Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2022	Dec. 31, 2021
Deferred Income Tax Assets [Abstract]
Employee Benefits and Compensation		$ 46.4	$ 44.5
Property Related		61.9	54.8
NOL Carryforwards		16.7	67.5
Deferred Tax Assets, Capital Loss Carryforwards		13.1	0.0
Tax Credit Carryforwards		548.7	509.1
Power Sales Agreements		13.7	16.5
Regulatory Liabilities		95.5	101.5
Other		28.1	18.0
Gross Deferred Income Tax Assets		824.1	811.9
Deferred Income Tax Asset Valuation Allowance		(60.2)	(69.0)
Total Deferred Income Tax Assets		763.9	742.9
Deferred Income Tax Liabilities [Abstract]
Property-Related		661.7	680.8
Regulatory Assets for Benefit Obligations		57.7	54.9
Unamortized Investment Tax Credits		30.0	30.5
Partnership Basis Differences		126.0	106.3
Regulatory Assets		10.7	14.3
Other		28.0	30.0
Total Deferred Income Tax Liabilities		922.0	924.7
Net Deferred Income Taxes	[1]	158.1	181.8
Deferred Tax Liabilities - Deferred Gain		$ 7.9	$ 7.9

[1] Recorded as a net Deferred Income Tax liability on the Consolidated Balance Sheet.