Debt and Credit Agreements (Details) (USD $) In Millions, unless otherwise specified				1 Months Ended	3 Months Ended			1 Months Ended			1 Months Ended
	Jun. 30, 2012 Letter of Credit	Feb. 29, 2012 Revolving credit facility	Jun. 30, 2012 Credit facility due June 2014	Mar. 30, 2012 Broken Bow and Crofton Bluffs Wind Projects	Mar. 31, 2012 Broken Bow and Crofton Bluffs Wind Projects	Jun. 30, 2012 Broken Bow and Crofton Bluffs Wind Projects Short-term debt	Jun. 30, 2012 Walnut Creek Project Letter of Credit	Dec. 31, 2011 Tapestry Wind, LLC Term Loan	Jun. 30, 2012 Big Sky Wind, LLC	Oct. 31, 2009 Big Sky Wind, LLC MW	Oct. 31, 2009 Big Sky Wind, LLC Vendor financing loan due 2014	Jun. 30, 2012 Midwest Generation LLC. Revolving credit facility	Apr. 17, 2012 Beginning December 31, 2012 through December 31, 2013 Broken Bow and Crofton Bluffs Wind Projects	Apr. 02, 2012 Beginning December 31, 2012 through December 31, 2013 Broken Bow and Crofton Bluffs Wind Projects	Apr. 17, 2012 Beginning December 31, 2013 through December 31, 2027 Broken Bow and Crofton Bluffs Wind Projects	Apr. 02, 2012 Beginning December 31, 2013 through December 31, 2027 Broken Bow and Crofton Bluffs Wind Projects
Debt Instrument [Line Items]
Construction loans				$ 79
Term of amortizing loan (in years)					15 years			10 years
Total generating capacity (in megawatts)										240
Final maturity											5 years
Investment in consolidated affiliate									135
Line of credit facility		564	100									500
Floating interest rate received				LIBOR
Variable rate, spread (as a percent)				2.875%
Variable rate, increase rate after third, sixth, ninth and twelfth years (as a percent)				0.125%
Long-term debt - Gross								214
Vendor financing, max borrowing amount											206
Letter of credit facilities				13.1
Working capital facilities				5.5
Forward starting interest rate swap agreements, rate percentages													0.7825%	0.8275%	2.7475%	2.96%
Proceeds from issuance of debt								97
Debt service coverage ratio					1.20
Construction loan, amount outstanding						9
Period of debt service coverage ratio					12 months
Standby letters of credit	165						40
Standby letters of credit expiring in 2012	59
Standby letters of credit expiring in 2013	75
Standby letters of credit expiring in 2014	3
Standby letters of credit expiring in 2017	10
Standby letters of credit expiring in 2018	18
Outstanding letters of credit						6
Cash collateral	$ 51