Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended				1 Months Ended									1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 02, 2013 Midwest Generation LLC	Dec. 31, 2012 Midwest Generation LLC MW	Dec. 31, 2011 Midwest Generation LLC	Dec. 31, 2010 Midwest Generation LLC	Dec. 31, 2009 Midwest Generation LLC	Dec. 31, 2012 Midwest Generation LLC Powerton, Joliet, Will County and Waukegan Stations of Midwest Generation LLC. MW	Dec. 31, 2012 Midwest Generation LLC Fisk and Waukegan Stations of Midwest Generation LLC. MW	Dec. 31, 2012 Viento Funding II, Inc.	Nov. 30, 2010 High Lonesome Mesa, LLC	Aug. 31, 2000 Powerton and Joliet Power Stations Midwest Generation LLC	Dec. 31, 2012 Powerton Station Midwest Generation LLC	Dec. 31, 2012 Joliet Units 7 and 8 Midwest Generation LLC	Dec. 31, 2012 Unsecured Notes	Feb. 28, 2011 Term Loan Viento Funding II, Inc.	Dec. 31, 2012 Term Loan due 2020 Term Loan Viento Funding II, Inc.	Dec. 31, 2011 Term Loan due 2020 Term Loan Viento Funding II, Inc.	Dec. 31, 2012 Bonds due 2017 Bonds High Lonesome Mesa, LLC	Dec. 31, 2011 Bonds due 2017 Bonds High Lonesome Mesa, LLC
Property, Plant and Equipment [Line Items]
Generating capacity (in megawatts)						4,619				4,314	305
Debt issuance														$ 1,147
Unsecured debt													81				3,700	227
Debt	1,438	4,912																	191	207	69	72
Long-term debt	1,056	4,912										191
Cash and cash equivalents, without contractual dividend restrictions	815
Cash and cash equivalents	888	1,221	948	680		95	213	295	237
Sale leaseback transaction lease period (years)															33 years 9 months	30 years
Lease payments not paid					76
Rent paid for sales leaseback transaction					7
Write off of unamortized deferred financing costs and debt discounts	$ 15