Income Taxes (Accounting for Uncertainty in Income Taxes) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 171,000,000		$ 153,000,000		$ 115,000,000
Tax positions taken during the current year, Increases	0		9,000,000		0
Tax positions taken during the current year, Decreases	0		0		0
Tax positions taken during a prior year, Increases	0		9,000,000		126,000,000
Tax positions taken during a prior year, Decreases	(12,000,000)		0		(80,000,000)
Decreases for settlements during the period	0		0		(8,000,000)
Decreases resulting from a lapse in statute of limitations	0		0		0
Balance at December 31	159,000,000		171,000,000		153,000,000
Unrecognized tax benefits that would impact effective tax rate	154,000,000		166,000,000
Reasonably possible reduction of unrecognized tax benefits	1,000,000
Accrued interest and penalties	65,000,000		51,000,000
Net after-tax interest and penalties recognized in income tax expense	8,000,000		10,000,000		19,000,000
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	200,000,000
Penalty for understatement of tax liability (as a percent)	40.00%
Midwest Generation LLC
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	44,000,000		44,000,000		0
Tax positions taken during the current year, Increases	0		0		0
Tax positions taken during the current year, Decreases	0		0		0
Tax positions taken during a prior year, Increases	0	[1]	0	[1]	44,000,000	[1]
Tax positions taken during a prior year, Decreases	0		0		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	44,000,000		44,000,000		44,000,000
Unrecognized tax benefits that would impact effective tax rate	41,000,000		41,000,000
Accrued interest and penalties	23,000,000		20,000,000
Net after-tax interest and penalties recognized in income tax expense	2,000,000		3,000,000		10,000,000
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

[1] Unrecognized tax benefits relate to tax positions taken in prior years and result from a review of Midwest Generation's deferred tax assets and liabilities.