Property, Plant and Equipment (Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010 projects
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 70		$ 47		$ 38
Accretion expense	5		5		2
Revisions	0		(1)		0
Liabilities added	5		19		8
Transfers out	0	[1]	0	[1]	(1)	[1]
Ending balance	80		70		47
Number of wind projects deconsolidated (in projects)					2
Number of projects consolidated (in projects)					1
Midwest Generation LLC
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	2		2		2
Accretion expense	0		1		0
Revisions	0		(1)		0
Ending balance	2		2		2
Fair value of legally restricted assets for purposes of settling AROs	$ 4

[1] Transfers out represents the deconsolidation of two wind projects and consolidation of one coal project effective January 1, 2010.