| Revenue | $3.3B | $4.2B | $3.7B | $4.9B | $3.6B | $4.6B | $4.0B | $5.2B |
| Return on regulatory assets | $1.0M | $145.0M | $167.0M | $3.2B | $0.0 | $42.0M | $201.0M | $3.8B |
| Other operating expense | $1.4B | $986.0M | $1.7B | $2.2B | $1.7B | $2.0B | $1.9B | $2.4B |
| INCOME BEFORE INTEREST AND INCOME TAX EXPENSE | $480.0M | $1.0B | $625.0M | $1.3B | $596.0M | $1.2B | $689.0M | $1.5B |
| Net Change in Cash and Restricted Cash | $1.3B | -$1.4B | $1.2B | -$973.0M | $1.1B | -$1.3B | $1.4B | -$1.5B |
| Cash disbursed for construction in progress | $1.2B | $1.1B | $1.2B | $1.2B | $1.3B | $1.0B | $1.3B | $1.2B |
| Property, plant, and equipment additions | $1.2B | $1.2B | $1.3B | $1.1B | $1.3B | $1.2B | $1.4B | $1.2B |
| Taxes other than income taxes and other – net | $782.0M | $857.0M | $832.0M | $947.0M | $894.0M | $953.0M | $963.0M | $1.0B |
| Other operations and maintenance | $967.0M | $965.0M | $910.0M | $952.0M | $923.0M | $967.0M | $964.0M | $940.0M |
| Cash Provided by (Used in) Investing Activity, Including Discontinued Operation | -$1.3B | -$1.3B | -$1.4B | -$1.2B | -$1.4B | -$1.1B | -$1.5B | -$936.0M |
| Comprehensive Income (Loss), Net of Tax, Attributable to Parent | $202.0M | $588.0M | $321.0M | $779.0M | $246.0M | $688.0M | $295.0M | $921.0M |
| Other Comprehensive Income (Loss) | $202.0M | $588.0M | $321.0M | $779.0M | $246.0M | $688.0M | $295.0M | $921.0M |
| Proceeds From Issuance Initial Public Offering | $0.0 | $0.0 | $0.0 | $1.3B | — | — | — | $776.0M |
| Issuance of common stock in connection with the Spin-Off and reclassification of net parent investment | — | — | — | $1.3B | — | — | — | $776.0M |
| Construction in progress additions | $41.0M | $14.0M | $5.0M | $416.0M | $0.0 | $378.0M | $74.0M | $732.0M |
| Cash Provided by (Used in) Financing Activity, Including Discontinued Operation | $1.3B | — | $1.3B | -$579.0M | $592.0M | -$698.0M | $1.4B | -$721.0M |
| Increase (Decrease) in Prepaid Expense | -$586.0M | $934.0M | -$927.0M | $554.0M | -$625.0M | $771.0M | -$764.0M | $691.0M |
| Repayments of Debt | $0.0 | $0.0 | $0.0 | $0.0 | — | — | — | $500.0M |
| Repayments of Short-Term Debt | $29.5M | $29.5M | $29.5M | $1.8B | — | — | — | $406.0M |
| Proceeds From Divestiture Of Interest In Joint Venture | — | — | — | $0.0 | — | — | — | $358.0M |
| Deferred taxes | — | $287.0M | — | $247.0M | $54.0M | $197.0M | $92.0M | $351.0M |
| Interest and Debt Expense | $303.0M | $322.0M | $322.0M | $331.0M | $318.0M | $323.0M | $324.0M | $326.0M |
| Dividends, Common Stock | $287.0M | $287.0M | $288.0M | $295.0M | $306.0M | $307.0M | $306.0M | $320.0M |
| Debt | $267.0M | $275.0M | $287.0M | $292.0M | $292.0M | $292.0M | $301.0M | $308.0M |
| Payments of Ordinary Dividends, Common Stock | $275.0M | $275.0M | $276.0M | $282.0M | $294.0M | $295.0M | $295.0M | $308.0M |
| Cash Dividends, Common Stock | $275.0M | $275.0M | $276.0M | $282.0M | $294.0M | $295.0M | $295.0M | $308.0M |
| Income Tax Expense | -$10.0M | $133.0M | $11.0M | $242.0M | $50.0M | $202.0M | $81.0M | $303.0M |
| Repayments of Short-Term Debt, Maturing in More than Three Months | — | — | — | $0.0 | — | — | — | $300.0M |
| Net Deferred Gain Loss | -$79.0M | -$176.0M | — | $84.0M | -$14.0M | $4.0M | $324.0M | $260.0M |
| Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount | $40.0M | $152.0M | $67.0M | $217.0M | $62.0M | $187.0M | $79.0M | $258.0M |
| Accounts payable | — | -$33.0M | — | -$140.0M | $175.0M | -$163.0M | $289.0M | -$220.0M |
| Increase (Decrease) In Corporate Alternative Minimum Tax (CAMT) Carry Forward | — | — | — | — | — | — | — | -$205.0M |
| Change in Accounts Receivable | — | $169.0M | $33.0M | $305.0M | -$357.0M | -$51.0M | $133.0M | $202.0M |
| Increase (Decrease) in Accounts Receivable | — | $169.0M | $33.0M | $305.0M | -$357.0M | -$51.0M | $133.0M | $202.0M |
| Gain on sale of interest in Mountain Valley Pipeline, LLC | — | — | — | $0.0 | — | — | — | $189.0M |
| Equity Method Investment Realized Gain Loss On Disposal | — | — | — | $0.0 | — | — | — | $189.0M |
| Cash Provided by (Used in) Operating Activity, Including Discontinued Operation | $1.3B | $392.0M | $1.3B | $837.0M | $2.0B | $504.0M | $1.5B | $174.0M |
| Interest Paid, Net | $385.0M | $137.0M | $415.0M | $149.0M | $411.0M | $166.0M | $421.0M | $168.0M |
| Interest Paid, Excluding Capitalized Interest, Operating Activity | $385.0M | $137.0M | $415.0M | $149.0M | $411.0M | $166.0M | $421.0M | $168.0M |
| Increase Decrease In Interest Payable Net | — | $140.0M | — | $137.0M | — | $126.0M | — | $143.0M |
| Membership and other income | $155.0M | $157.0M | $159.0M | $201.0M | $214.0M | $208.0M | $214.0M | $143.0M |
| Pension and other postretirement benefits contributions | -$18.0M | -$115.0M | -$99.0M | -$148.0M | -$136.0M | -$139.0M | -$159.0M | -$118.0M |
| Change in Other Liabilities | — | — | -$31.0M | $9.0M | $54.0M | $14.0M | $86.0M | -$112.0M |
| Other liabilities | — | — | -$31.0M | $9.0M | $54.0M | $14.0M | $86.0M | -$112.0M |
| Increase Decrease In Deferred Liabilities | $105.0M | $209.0M | $75.0M | $299.0M | — | — | $267.0M | $110.0M |
| Payments For Proceeds From Removal Costs | $107.0M | $121.0M | $139.0M | $107.0M | $144.0M | $84.0M | $146.0M | $106.0M |
| Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount | $5.0M | $39.0M | $24.0M | $55.0M | $13.0M | $47.0M | $24.0M | $64.0M |
| Contract with Customer, Liability, Unbilled Revenue Accruals And Deferrals | -$18.8M | -$18.8M | -$18.8M | $7.0M | — | — | — | $45.0M |
| Increase (Decrease) in Income Taxes Receivable | $0.0 | $0.0 | $144.0M | -$26.0M | -$118.0M | — | — | $43.0M |
| Additions / (Releases) | $40.0M | $27.0M | -$214.0M | $4.0M | -$1.0M | -$5.0M | -$497.0M | $39.0M |
| Increase (Decrease) in Other Receivables | -$68.0M | -$47.0M | — | -$28.0M | -$101.0M | -$44.0M | $206.0M | -$37.0M |
| Issuance of Common Stock (Employee Equity Plans) | $30.0M | $26.0M | $29.0M | $29.0M | $30.0M | $29.0M | $29.0M | $27.0M |
| Allowance for Funds Used During Construction, Equity | $10.0M | $8.0M | $9.0M | $18.0M | $18.0M | $16.0M | $17.0M | $22.0M |
| Shares Issued for Share-Based Compensation | $15.0M | $15.0M | $15.0M | $16.0M | $16.0M | $17.0M | $15.0M | $18.0M |
| Other interest expense | $36.0M | $47.0M | $35.0M | $39.0M | $26.0M | $31.0M | $23.0M | $18.0M |
| Net interest and investment income (loss) | $11.0M | $17.0M | $16.0M | $16.0M | $17.0M | $18.0M | $12.0M | $18.0M |
| Proceeds From Issuance Of Shares Under Incentive And Share Based Compensation Plans Including Stock Options | $15.0M | $15.0M | $15.0M | $16.0M | $16.0M | $17.0M | $15.0M | $18.0M |
| Effective Income Tax Rate Reconciliation, Deductible Expense Amortization Of Excess Deferred Income Taxes, Amount | $50.8M | $50.8M | $50.8M | $12.0M | — | — | — | $16.0M |
| Other investing activities, net | $12.0M | $9.0M | $7.0M | $14.0M | $14.0M | $8.0M | $13.0M | $15.0M |
| Distributions of earnings from equity method investees | $2.0M | $13.0M | $20.0M | $16.0M | $17.0M | $18.0M | $19.0M | $15.0M |
| Payment for (Proceeds from) Other Investing Activity | $12.0M | $9.0M | $7.0M | $14.0M | $14.0M | $8.0M | $13.0M | $15.0M |
| Total deferred gains (losses) | -$78.0M | -$38.0M | -$15.0M | $100.0M | -$80.0M | -$15.0M | $246.0M | $13.0M |
| Cost of removal | $8.0M | $10.0M | — | $11.0M | $9.0M | $10.0M | $6.0M | $13.0M |
| Other Noncash Income (Expense) | — | $71.0M | — | $18.0M | $81.0M | $60.0M | — | -$13.0M |
| Included in regulatory assets and liabilities | -$1.8M | -$1.8M | -$1.8M | $0.0 | — | — | — | $12.0M |
| MTA Surcredit amortization, net of federal income taxes | -$2.0M | $0.0 | -$1.0M | -$13.0M | -$7.0M | -$5.0M | -$9.0M | -$10.0M |
| Income Taxes Paid, Net | $1.0M | $2.0M | $1.0M | -$33.0M | -$148.0M | — | — | -$7.0M |
| Income Taxes Paid, Net | $1.0M | $2.0M | $1.0M | -$33.0M | -$148.0M | — | — | -$7.0M |
| New issues (in shares) | — | — | 7M | 6.3M | — | — | — | 7M |
| Allowance for uncollectible accounts, net of regulatory recovery | -$6.3M | -$6.3M | -$6.3M | -$13.0M | — | — | — | -$6.0M |
| Payment for Pension and Other Postretirement Benefits | $5.0M | $12.0M | $5.0M | $5.0M | $4.0M | $11.0M | $41.0M | $6.0M |
| Operating Lease Payments | $4.0M | $6.0M | $52.0M | $4.0M | $4.0M | $7.0M | $55.0M | $5.0M |
| Operating Lease Payments | $4.0M | $6.0M | $52.0M | $4.0M | $4.0M | $7.0M | $55.0M | $5.0M |
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements | -$3.0M | -$2.0M | -$3.0M | -$4.0M | -$1.0M | $2.0M | -$2.0M | -$4.0M |
| Effective Income Tax Rate Reconciliation, Other Adjustments, Amount | -$1.0M | -$2.0M | -$4.0M | $1.0M | -$3.0M | -$8.0M | -$2.0M | $4.0M |
| Other Comprehensive Income (Loss), Net of Tax | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| OCI Attributable to Parent | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| OCI Defined Benefit Plans Adjustment (Net of Tax) Parent | $0.0 | $0.0 | -$11.0M | $12.0M | $0.0 | $0.0 | $2.0M | $3.0M |
| OCI defined benefit plans | $0.0 | $0.0 | -$11.0M | $12.0M | $0.0 | $0.0 | $2.0M | $3.0M |
| Other comprehensive income (loss), after-tax | — | — | — | -$12.0M | — | — | — | -$3.0M |
| Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent | $0.0 | $0.0 | -$11.0M | $12.0M | $0.0 | $0.0 | $2.0M | $3.0M |
| Other comprehensive income (loss) | — | — | — | -$12.0M | — | — | — | -$3.0M |
| Other comprehensive income/(loss) | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| Other Comprehensive Income (Loss), Net of Tax, Attributable to Parent | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| OCI Net of Tax, Parent | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| OCI Attributable to Parent | $0.0 | $0.0 | $11.0M | -$12.0M | $0.0 | $0.0 | -$2.0M | -$3.0M |
| Realized and unrealized gains (losses): Included in income (loss) | -$2.0M | -$1.0M | -$3.0M | -$2.0M | -$1.0M | $2.0M | -$2.0M | -$2.0M |
| Other Increase (Decrease) in Environmental Liabilities | -$9.0M | -$12.0M | -$16.0M | -$10.0M | -$4.0M | — | -$12.0M | -$2.0M |
| Issuance of vested restricted stock units | — | $11.0M | — | — | $3.0M | $2.0M | — | $2.0M |
| Add - Incremental shares associated with contingently issuable shares (in shares) | 1.2M | 1.3M | 1.3M | 1.2M | 1.3M | 1.2M | 1.3M | 1.4M |
| Transfers out of Level 3 | $0.0 | -$5.0M | $0.0 | $0.0 | — | — | $0.0 | -$1.0M |
| Taxes other than income taxes payable | -$1.0M | — | — | $12.0M | -$25.0M | $9.0M | $37.0M | -$1.0M |
| Payments Non Utility Construction Expenditures | $0.0 | $0.0 | $1.0M | $0.0 | $1.0M | — | — | $1.0M |
| Payments of Debt Issuance Costs | $22.0M | $2.0M | $18.0M | $1.0M | $0.0 | $2.0M | $12.0M | $1.0M |
| Rate adjustment mechanism | — | — | — | — | — | — | — | $500.0K |
| Number of registrants | — | — | — | 2.00 | — | — | — | 2.00 |
| Number of regulated utility subsidiaries | — | — | — | 2.00 | — | — | — | 2.00 |
| Effective tax rates | -$0.1 | $0.2 | — | $0.2 | $0.2 | $0.2 | — | $0.2 |
| Income taxes at U.S. statutory rate of 21% | $0.2 | $0.2 | — | $0.2 | $0.2 | $0.2 | — | $0.2 |
| State income taxes – net of federal income tax benefit | $0.0 | $0.1 | $0.0 | $0.1 | $0.0 | $0.1 | $0.0 | $0.1 |
| Other customer-provided capital rate (in percent) | — | — | 5.9% | 4.8% | — | — | 4.8% | 4.7% |
| Weighted average interest rate of commercial paper (in percent) | — | — | 4.7% | 4.6% | — | — | 3.9% | 4.0% |
| Effective Income Tax Rate Reconciliation, Deductible Expense Amortization Of Excess Deferred Income Taxes, Percent | — | — | 9.5% | 1.1% | — | — | 2.0% | 1.3% |
| Cost of removal | 4.4% | 1.4% | 1.1% | 1.0% | 3.0% | 1.1% | 1.4% | 1.1% |
| MTA Surcredit amortization, net of federal income taxes | -1.0% | 0.0% | -0.1% | -1.3% | -2.5% | -0.6% | -1.3% | -0.8% |
| Allowance for uncollectible accounts, net of regulatory recovery | 0.5% | -1.4% | -1.2% | -1.2% | -2.9% | -1.6% | -1.7% | -0.5% |
| ROU Asset Obtained - Operating Lease | $0.0 | $0.0 | $3.0M | $8.0M | $0.0 | $3.0M | $35.0M | — |
| Stock options excluded as their inclusion would be anti-dilutive (in shares) | $0.0 | $0.0 | — | $0.0 | — | $0.0 | — | $0.0 |
| Capitalization, Forward Sale Price | $24.16 | $24.16 | $24.16 | — | — | — | — | — |
| Nontaxable or nondeductible items | — | — | -0.1% | -0.1% | — | — | 0.2% | 0.0% |
| Changes in unrecognized tax benefits | — | — | -0.1% | 0.1% | — | — | 0.0% | 0.0% |
| Changes in unrecognized tax benefits | — | — | -$5.0M | $1.0M | — | — | — | $0.0 |
| Termination cost | — | — | — | $0.0 | $0.0 | $0.0 | — | $0.0 |
| Other income, net of taxes | $0.0 | $0.0 | $0.0 | — | — | — | — | — |
| Borrowing under term loan | — | $0.0 | $500.0M | $200.0M | — | — | — | $0.0 |
| Divestiture of businesses, net of cash transferred | $0.0 | $0.0 | $0.0 | $45.0M | — | — | — | $0.0 |
| Issuance of term loan | $125.0M | $125.0M | $125.0M | — | — | — | — | — |
| Right Of Use Asset Obtained In Exchange For Operating Lease Liability | $0.0 | $0.0 | $3.0M | $8.0M | $0.0 | $3.0M | $35.0M | — |
| Unconditional charitable contribution, accrued amount | $12.0M | — | $12.0M | $12.0M | — | — | — | — |
| Unrecognized tax benefits, increase (decrease) resulting from prior period tax positions | -$750.0K | -$750.0K | -$750.0K | — | — | — | — | — |
