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Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Jun. 30, 2023
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | $ 713.0 | $ 527.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 501.0 |  |  |
Regulatory Liabilities | 1,446.0 | 1,623.0 |  |
Deferred Purchased Power Costs | 5.0 | 23.0 | $ 62.0 |
Removal Costs [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
Regulatory Liabilities | 1,173.0 | 1,136.0 |  |
Deferred Income Tax Charge [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
Regulatory Liabilities | 177.0 | 194.0 |  |
Asset Retirement Obligation Costs [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Liabilities | 4.0 | 7.0 |  |
Other Regulatory Assets (Liabilities) [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 14.0 |  |  |
Regulatory Liabilities | 96.0 | 98.0 |  |
Regulatory Assets Earning a Rate of Return at the Approved Rate | 82.0 |  |  |
RemainingAmountsOfRegulatoryLiabilitesForWhichNoInterstAccruesDuringRecoveryPeriodIsProvided | 14.0 |  |  |
Deferred Derivative Gain (Loss) [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets |  | 195.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
Regulatory Liabilities | 0.0 |  |  |
Earning a return [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Liabilities | 1,432.0 |  |  |
Boardman refund [Domain] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Liabilities |  |  |  | $ 6.5
Other Regulatory Assets Earning a Return [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 58.0 |  |  |
Earning a return [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 212.0 |  |  |
Deferred Derivative Gain (Loss) [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 0.0 | 1.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 206.0 |  |  |
Pension and Other Postretirement Plans Costs [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 0.0 | 95.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 104.0 |  |  |
Loss on Reacquired Debt [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 0.0 | 21.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 20.0 |  |  |
Environmental Restoration Costs [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 0.0 |  |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 139.0 | 133.0 |  |
Other Regulatory Assets (Liabilities) [Member] |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 90.0 | 94.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 32.0 |  |  |
February2021IceStormAndDamageDeferral |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 67.0 | 74.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
PowerCostAdjustmentMechanismDeferral |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 16.0 | 28.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
2020LaborDayWildfireDeferral |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 28.0 | 31.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
COVID-19Deferral |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 14.0 | 22.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0.0 |  |  |
Wildfire mitigation |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Regulatory Assets | 29.0 | $ 28.0 |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 0.0 |  |  |

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- Definition Costs of power used for operations in prior periods that were expected to be recovered from customers in future periods. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (a) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 + Details
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- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481849/980-340-55-2 + Details
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