Income Tax Expense - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets [Abstract]
Employee Benefits and Compensation		$ 104.6	$ 105.4
Property Related		117.8	126.6
NOL Carryforwards		185.6	186.4
Tax Credit Carryforwards		227.4	164.8
Power Sales Agreements		59.3	73.0
Other		46.9	21.8
Gross Deferred Tax Assets		741.6	678.0
Deferred Tax Asset Valuation Allowance		(43.0)	(31.6)
Total Deferred Tax Assets		698.6	646.4
Deferred Tax Liabilities [Abstract]
Property Related		1,094.7	1,053.0
Regulatory Asset for Benefit Obligations		91.9	89.4
Unamortized Investment Tax Credits		33.3	26.0
Partnership Basis Differences		50.9	47.8
Other		11.9	10.0
Total Deferred Tax Liabilities		1,282.7	1,226.2
Net Deferred Income Taxes	[1]	$ 584.1	$ 579.8

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