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