Impairment, Disposition and Assets and Liabilities Held for Sale (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Other Asset Impairment Charges	$ 11.2	$ 0.0
Assets Held for Sale	9.7		$ 1,951.2
Liabilities Held for Sale	3.5		235.9
Generation And Marketing [Member]
Cash Proceeds from Sale of Plants Gross	2,200.0
Cash Proceeds from Disposition of Plants, Net	1,200.0
Pre-tax Gain on Sale of Plants	227.0
Maximum Potential Loss Related to Guarantee	34.0
Income from Continuing Operations before Income Tax Expense and Equity Earnings of the Plants Sold	47.0	$ 112.0
Fuel	6.9		145.5
Materials and Supplies	0.1		49.4
Property, Plant and Equipment - Net	0.8		1,756.2
Other Classes of Assets That Are Not Major	1.9		0.1
Assets Held for Sale	9.7		1,951.2
Long-term Debt	0.0		134.8
Waterford Plant Upgrade Liability	0.0		52.2
Asset Retirement Obligations	1.9		36.7
Other Classes of Liabilities That Are Not Major	1.6		12.2
Liabilities Held for Sale	3.5		$ 235.9
Merchant Coal-Fired Generation Assets [Member] | Generation And Marketing [Member]
Other Asset Impairment Charges	4.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Merchant Coal-Fired Generation Assets [Member] | Generation And Marketing [Member]
Other Asset Impairment Charges	$ 7.0