Asset Impairments and Other Charges (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011 MW
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment and other charges	$ 464,000,000	[1]	$ 28,000,000	[1]	$ 714,000,000	[1]
Impairment charges	464,000,000
Other charges	(2,000,000)		13,000,000		74,000,000
Reduced projected capacity for development projects (in mw)					1,300
Charges from reduction of development projects					34,000,000
Midwest Generation Plant
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges	466,000,000		0		640,000,000
American Bituminous
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges	0		15,000,000		0
Aggregate carrying value of long-lived assets			49,000,000
Fair value of long lived assets			34,000,000
Storm Lake Project and Four Small Wind Projects in Minnesota
Impaired Long-Lived Assets Held and Used [Line Items]
Number of projects (project)					4
Impairment charges					30,000,000
Aggregate carrying value of long-lived assets					53,000,000
Fair value of long lived assets					23,000,000
Number of asset groups, used to determine impairment					5
Midwest Generation LLC [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment and other charges	465,000,000		14,000,000		653,000,000
Impairment charges	464,000,000				640,000,000
Midwest Generation LLC [Member] | Will County Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges	464,000,000
Property, Plant, and Equipment, Fair Value Disclosure	0
Midwest Generation LLC [Member] | Fisk Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges					115,000,000
Midwest Generation LLC [Member] | Crawford Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges					186,000,000
Midwest Generation LLC [Member] | Waukegan Station
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges					339,000,000
Minimum | Level 3 | Midwest Generation LLC [Member] | Will County Station
Impaired Long-Lived Assets Held and Used [Line Items]
Fair value per kilowatt hour	0
Maximum | Level 3 | Midwest Generation LLC [Member] | Will County Station
Impaired Long-Lived Assets Held and Used [Line Items]
Fair value per kilowatt hour	$ 169

[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.