Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Deferred Income Tax Assets [Abstract]
Employee Benefits and Compensation		$ 62.2	$ 65.9
Property Related		95.2	104.3
NOL Carryforwards		86.1	99.1
Tax Credit Carryforwards		349.8	294.3
Power Sales Agreements		27.5	35.0
Regulatory Liabilities		113.4	117.7
Other		25.1	33.3
Gross Deferred Income Tax Assets		759.3	749.6
Deferred Income Tax Asset Valuation Allowance		(66.5)	(60.0)
Total Deferred Income Tax Assets		692.8	689.6
Deferred Income Tax Liabilities [Abstract]
Property-Related		752.5	758.3
Regulatory Assets for Benefit Obligations		61.0	61.4
Unamortized Investment Tax Credits		32.2	32.8
Partnership Basis Differences		40.8	34.9
Regulatory Assets		29.9	32.0
Other		0.0	0.7
Total Deferred Income Tax Liabilities		916.4	920.1
Net Deferred Income Taxes	[1]	$ 223.6	$ 230.5

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