Asset Impairments, Lease Terminations and Other (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended			3 Months Ended	12 Months Ended										3 Months Ended
	Dec. 31, 2011 MW	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Homer City	Dec. 31, 2010 Homer City	Dec. 31, 2009 Homer City	Dec. 31, 2011 Midwest Generation LLC.	Dec. 31, 2011 Midwest Generation LLC.	Dec. 31, 2010 Midwest Generation LLC.	Dec. 31, 2009 Midwest Generation LLC.	Dec. 31, 2011 Fisk Station	Dec. 31, 2011 Crawford Station	Dec. 31, 2011 Waukegan Station	Dec. 31, 2011 Wind Projects	Dec. 31, 2010 Wind Projects	Dec. 31, 2009 Wind Projects	Dec. 31, 2011 Storm Lake Project and Four Small Wind Projects in Minnesota [Member] groups	Dec. 31, 2010 Powerton Station Capitalized Engineering and Other Costs	Dec. 31, 2011 Edison Mission Marketing and Trading, Inc [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment					$ 1,032	$ 0	$ 0	$ 640	$ 640	$ 40	$ 0	$ 115	$ 186	$ 339				$ 30
Lease termination	0	0	887
Impairment and other charges															64	0	0
Other	36	7	3
Asset impairments, lease terminations and other	1,772	47	890
Cash	1,469	1,389	1,673	3,916	84
Restricted deposits					27
Inventory	624	568			105
Other assets					43
Total assets	48,039	45,530	41,444		259
Accounts payable and accrued liabilities					30
Pensions and other postretirement benefits	2,912	2,260			49
Other liabilities					13
Total liabilities	36,953	34,036			92
Net assets					167
Intercompany accounts receivable due from Homer City																				38
Write off of intangible asset																			40
Aggregate carrying value of long-lived assets																		53
Number of asset groups, used to determine impairment																		5
Fair value of asset groups																		23
Reduced projected capacity for development projects (in mw)	1,300
Charges from reduction of development projects	$ 34