Income Tax Expense - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets [Abstract]
Employee Benefits and Compensation		$ 105.4	$ 102.2
Property Related		126.6	102.7
NOL Carryforwards		186.4	156.5
Tax Credit Carryforwards		164.8	95.7
Power Purchase Agreements		73.0	51.8
Other		21.8	17.0
Gross Deferred Tax Assets		678.0	525.9
Deferred Tax Asset Valuation Allowance		(31.6)	(22.1)
Total Deferred Tax Assets		646.4	503.8
Deferred Tax Liabilities [Abstract]
Property Related		1,053.0	848.8
Regulatory Asset for Benefit Obligations		89.4	89.9
Unamortized Investment Tax Credits		26.0	10.3
Partnership Basis Differences		47.8	41.9
Other		10.0	16.1
Total Deferred Tax Liabilities		1,226.2	1,007.0
Net Deferred Income Taxes		579.8	503.2
Net Current Deferred Tax Assets	[1]	0.0	7.5
Net Long-Term Deferred Tax Liabilities		$ 579.8	$ 510.7

[1] For discussion of classification of deferred income taxes see Note 1. Operations and Significant Accounting Policies - New Accounting Standards - Balance Sheet Classification of Deferred Taxes.