Environmental Developments (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended			12 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2011 Greenhouse Gas Litigation Developments companies	Oct. 31, 2009 Kivalina case	Apr. 30, 2011 Southern California Edison Company	Dec. 31, 2011 Edison Mission Group Inc.	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 Joliet unit 6	Dec. 31, 2011 Waukegan Station	Dec. 31, 2011 Homer City	Dec. 31, 2010 Homer City	Dec. 31, 2009 Homer City	Dec. 31, 2011 Homer City Minimum	Dec. 31, 2011 Homer City Maximum
Environmental developments
Impairment charges				$ 1,700	$ 640	$ 640	$ 40	$ 0		$ 339	$ 1,032	$ 0	$ 0
Estimated capital costs for SO2 reduction						628								700	750
Estimated cost of retrofitting									75	160
Electricity requirements required to be purchased as per the law enacted by legislature (as a percent)			33.00%
Number of power companies against whom public nuisance claims are dismissed	5
Damages sought in Kivalina case		$ 400