Income Taxes (Accounting for Uncertainty in Income Taxes) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 159,000,000	$ 171,000,000	$ 153,000,000
Tax positions taken during the current year, Increases	0	0	9,000,000
Tax positions taken during the current year, Decreases	0	0	0
Tax positions taken during a prior year, Increases	0	0	9,000,000
Tax positions taken during a prior year, Decreases	0	(12,000,000)	0
Decreases for settlements during the period	0	0	0
Decreases resulting from a lapse in statute of limitations	0	0	0
Balance at December 31	159,000,000	159,000,000	171,000,000
Unrecognized tax benefits that would impact effective tax rate	154,000,000	154,000,000
Reasonably possible reduction of unrecognized tax benefits	66,000,000
Accrued interest and penalties	76,000,000	65,000,000
Net after-tax interest and penalties recognized in income tax expense	6,000,000	8,000,000	10,000,000
IRS Examination
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Estimated potential future federal tax payment as of the balance sheet date, including interest and penalties, based on a proposed adjustment	207,000,000
Interest From Income Tax Examination	59,000,000
Penalties From Income Tax Examination	42,000,000
Non-controlling Interest
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits that would impact effective tax rate		41,000,000
Midwest Generation LLC [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	44,000,000	44,000,000	44,000,000
Unrecognized Tax Benefits, Period Increase (Decrease)	0	0	0
Balance at December 31	44,000,000	44,000,000	44,000,000
Unrecognized tax benefits that would impact effective tax rate	41,000,000
Reasonably possible reduction of unrecognized tax benefits	36,000,000
Accrued interest and penalties	26,000,000	23,000,000
Net after-tax interest and penalties recognized in income tax expense	2,000,000	2,000,000	3,000,000
Homer City
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Tax deduction due to change in tax treatment of entity		$ 1,000,000,000