Asset Impairments and Other Charges (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011 MW	Dec. 31, 2012		Dec. 31, 2011 MW		Dec. 31, 2010
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment and other charges		$ 28	[1]	$ 714	[1]	$ 44	[1]
Impairment charges	704
Other charges		13		10		4
Reduced projected capacity for development projects (in mw)	1,300			1,300
Charges from reduction of development projects				34
Wind Projects
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment and other charges		0		64		0
Midwest Generation LLC
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges		0	[2]	640	[2]	40	[2]
American Bituminous
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges		15		0		0
Aggregate carrying value of long-lived assets		49
Fair value of long lived assets		34
Storm Lake Project and Four Small Wind Projects in Minnesota
Impaired Long-Lived Assets Held and Used [Line Items]
Number of projects (project)	4			4
Impairment charges				30
Aggregate carrying value of long-lived assets	53			53
Fair value of long lived assets	23			23
Number of asset groups, used to determine impairment	5			5
Midwest Generation LLC
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment and other charges		14	[1]	653	[1]	48	[1]
Impairment charges	640	0		640		40
Other charges		14		13		8
Midwest Generation LLC | Fisk Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges				115
Midwest Generation LLC | Crawford Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges				186
Midwest Generation LLC | Waukegan Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges				$ 339

[1] The fair value of long-lived assets as determined using the discounted cash flow models discussed below qualify as Level 3 in the fair value hierarchy.

[2] EME and Midwest Generation each recorded impairment charges on the Midwest Generation plants in 2011 and 2010. For an explanation of these charges, see the discussion below under Midwest Generation.