Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Deferred Income Tax Assets [Abstract]
Employee Benefits and Compensation		$ 67.6	$ 49.9
Property Related		61.4	76.9
NOL Carryforwards		60.7	63.2
Tax Credit Carryforwards		455.7	395.5
Power Sales Agreements		20.1	23.7
Regulatory Liabilities		107.7	116.9
Other		22.4	23.4
Gross Deferred Income Tax Assets		795.6	749.5
Deferred Income Tax Asset Valuation Allowance		(69.9)	(70.0)
Total Deferred Income Tax Assets		725.7	679.5
Deferred Income Tax Liabilities [Abstract]
Property-Related		691.5	713.4
Regulatory Assets for Benefit Obligations		71.5	54.5
Unamortized Investment Tax Credits		31.1	31.6
Partnership Basis Differences		86.7	49.4
Regulatory Assets		32.6	35.4
Other		8.0	8.0
Total Deferred Income Tax Liabilities		921.4	892.3
Net Deferred Income Taxes	[1]	$ 195.7	$ 212.8

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