Income Tax Expense - Deferred Tax Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred Tax Assets [Abstract]
Employee Benefits and Compensation	$ 102.2	$ 66.3
Property Related	102.7	82.2
NOL Carryforwards	156.5	112.8
Tax Credit Carryforwards	95.7	55.1
Power Purchase Agreements	51.8	0
Other	17.0	16.9
Gross Deferred Tax Assets	525.9	333.3
Deferred Tax Asset Valuation Allowance	(22.1)	(8.0)
Total Deferred Tax Assets	503.8	325.3
Deferred Tax Liabilities [Abstract]
Property Related	848.8	656.2
Regulatory Asset for Benefit Obligations	89.9	58.7
Unamortized Investment Tax Credits	10.3	11.1
Partnership Basis Differences	41.9	36.7
Other	16.1	22.7
Total Deferred Tax Liabilities	1,007.0	785.4
Net Deferred Income Taxes	503.2	460.1
Net Current Deferred Tax Assets	7.5	19.0
Net Long-Term Deferred Tax Liabilities	$ 510.7	$ 479.1