Quarterly Financial Data (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended									12 Months Ended			3 Months Ended										12 Months Ended			3 Months Ended		12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Midwest Generation LLC		Sep. 30, 2012 Midwest Generation LLC	Jun. 30, 2012 Midwest Generation LLC	Mar. 31, 2012 Midwest Generation LLC	Dec. 31, 2011 Midwest Generation LLC		Sep. 30, 2011 Midwest Generation LLC	Jun. 30, 2011 Midwest Generation LLC	Mar. 31, 2011 Midwest Generation LLC	Dec. 31, 2012 Midwest Generation LLC	Dec. 31, 2011 Midwest Generation LLC	Dec. 31, 2010 Midwest Generation LLC	Sep. 30, 2012 Homer City	Dec. 31, 2011 Homer City	Dec. 14, 2012 Homer City
Quarterly Financial Information [Line Items]
Operating revenues	$ 280	$ 340	$ 324	$ 343	$ 381		$ 437	$ 400	$ 435	$ 1,287	$ 1,653	$ 1,788	$ 193		$ 253	$ 213	$ 233	$ 289		$ 366	$ 280	$ 351	$ 892	$ 1,286	$ 1,479
Operating income (loss)	(102)	(78)	(100)	(48)	(717)		30	(44)	33	(328)	(698)	194	(1,437)	[1]	(39)	(88)	(33)	(607)		76	(47)	62	(1,597)	(516)	288
Provision (benefit) for income taxes										160	(441)	(16)	(23)		(7)	(27)	(5)	(231)		38	(11)	32	(62)	(172)	142
Income (loss) from continuing operations	(578)	(86)	(75)	(58)	(431)		18	(26)	(8)	(797)	(447)	81
Income (loss) from operations of discontinued subsidiaries, net of tax	17	(76)	(29)	(24)	(629)		15	(6)	(12)	(112)	(632)	82
Net Income (Loss)	(561)	(162)	(104)	(82)	(1,060)	[2]	33	(32)	(20)	(909)	(1,079)	163	(1,401)		(12)	(42)	(9)	(357)	[3]	57	(18)	48	(1,464)	(270)	215
Impairment charges					704													640					0	640	40		1,032
Impairment charges, after tax					424													386								68	623	53
Valuation allowance										$ 438	$ 0	$ 0											$ 1,899	$ 0	$ 0

[1] Reflects a $1.4 billion pre-tax charge for a valuation allowance recorded by Midwest Generation on its note receivable from EME. For more information, see Note 15—Related Party Transactions.

[2] Reflects $704 million pre-tax ($424 million after tax) of asset impairment charges. For more information, see Note 13—Asset Impairments and Other Charges. In addition, the amount reflects a $1,032 million pre-tax ($623 million after tax) impairment charge related to Homer City. For more information, see Note 14—Discontinued Operations.

[3] Reflects a $640 million pre-tax ($386 million, after tax) impairment charge. For more information, see Note 13—Asset Impairments and Other Charges.