Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Deferred Income Tax Assets [Abstract]
Employee Benefits and Compensation		$ 49.9	$ 62.2
Property Related		76.9	95.2
NOL Carryforwards		63.2	86.1
Tax Credit Carryforwards		395.5	349.8
Power Sales Agreements		23.7	27.5
Regulatory Liabilities		116.9	113.4
Other		23.4	25.1
Gross Deferred Income Tax Assets		749.5	759.3
Deferred Income Tax Asset Valuation Allowance		(70.0)	(66.5)
Total Deferred Income Tax Assets		679.5	692.8
Deferred Income Tax Liabilities [Abstract]
Property-Related		713.4	752.5
Regulatory Assets for Benefit Obligations		54.5	61.0
Unamortized Investment Tax Credits		31.6	32.2
Partnership Basis Differences		49.4	40.8
Regulatory Assets		35.4	29.9
Other		8.0	0.0
Total Deferred Income Tax Liabilities		892.3	916.4
Net Deferred Income Taxes	[1]	$ 212.8	$ 223.6

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