Debt and Credit Agreements (Project Financings) (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended	3 Months Ended			1 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended											1 Months Ended								1 Months Ended				0 Months Ended	1 Months Ended		0 Months Ended			1 Months Ended				1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 30, 2012 Broken Bow and Crofton Bluffs Wind Projects	Mar. 31, 2012 Broken Bow and Crofton Bluffs Wind Projects	Dec. 31, 2012 Broken Bow Wind, LLC	Dec. 31, 2012 Crofton Bluffs	Dec. 31, 2011 Tapestry Wind, LLC	Dec. 31, 2012 Tapestry Wind, LLC		Jul. 31, 2011 Walnut Creek Energy	Dec. 31, 2012 Walnut Creek Energy	Jul. 31, 2011 WCEP Holdings, LLC	Dec. 31, 2012 WCEP Holdings, LLC	Dec. 31, 2012 Midwest Generation LLC MW	Dec. 31, 2011 Midwest Generation LLC	Dec. 31, 2012 Laredo Ridge		Jul. 31, 2010 Laredo Ridge	Dec. 31, 2012 Cedro Hill Project		Mar. 31, 2010 Cedro Hill Project	Oct. 31, 2009 Big Sky Wind, LLC MW	Dec. 31, 2011 Working Capital Facility Tapestry Wind, LLC	Jul. 31, 2010 Working Capital Facility Laredo Ridge	Mar. 31, 2010 Working Capital Facility Cedro Hill Project	Dec. 31, 2012 Letter of Credit Tapestry Wind, LLC	Dec. 31, 2012 Letter of Credit Laredo Ridge	Jul. 31, 2010 Letter of Credit Laredo Ridge	Dec. 31, 2012 Letter of Credit Cedro Hill Project	Mar. 31, 2010 Letter of Credit Cedro Hill Project	Dec. 31, 2011 Term Loan Tapestry Wind, LLC	Dec. 31, 2012 Term Loan Tapestry Wind, LLC	Jul. 31, 2011 Term Loan Walnut Creek Energy	Jul. 31, 2011 Term Loan WCEP Holdings, LLC	Mar. 18, 2010 Term Loan Laredo Ridge	Mar. 18, 2011 Term Loan Laredo Ridge	Dec. 31, 2012 Term Loan Laredo Ridge	Dec. 22, 2010 Term Loan Cedro Hill Project	Dec. 31, 2012 Term Loan Cedro Hill Project	Mar. 31, 2010 Term Loan Cedro Hill Project	Dec. 31, 2011 Debt Service Reserve Letter of Credit Tapestry Wind, LLC	Dec. 31, 2011 Project Letter of Credit Tapestry Wind, LLC	Jul. 31, 2010 Construction Loan Laredo Ridge	Mar. 31, 2010 Construction Loan Cedro Hill Project	Oct. 31, 2009 Vendor financing loan due 2014 Big Sky Wind, LLC	Dec. 31, 2012 Vendor financing loan due 2014 Big Sky Wind, LLC	Dec. 31, 2012 Letter of Credit	Dec. 31, 2012 Letter of Credit Walnut Creek Project
Debt Instrument [Line Items]
Date of issuance			March 30, 2012				December 2011			July 2011								July 2010			March 2010															March 18, 2011		December 22, 2010							October 2009
Construction loans			$ 79							$ 495																							$ 442	$ 53
Term of amortizing loan (in years)				15 years			10 years																10 years								10 years										10 years
Letter of credit facilities			13								122												8																		12	8
Working capital facilities			6
Floating interest rate received			LIBOR				LIBOR			LIBOR		LIBOR																							LIBOR			LIBOR								LIBOR
Variable rate, spread (as a percent)			2.88%				2.50%			2.25%		4.00%																								2.75%				3.00%						3.50%
Variable rate, increase rate after every three years (as a percent)			0.13%							0.25%																										0.13%				0.13%
Construction loan, amount outstanding					52	27					330		52																			210
Outstanding letters of credit					10	3																				20
Long-term Debt, Gross																															214															222
Rate increase on every fourth anniversary							0.125%
Proceeds from Issuance of Debt																															97
Standby letters of credit																																															163	30
Debt instrument, face amount																								3	4			9		10													75	135
Long-term debt	1,056	4,912																									9		10								71		125
Variable rate, increase rate after every thirteen years (as a percent)																																								0.25%
Generating capacity (in megawatts)														4,619								240
Debt maturity period											10 years																									15 years		15 years							5 years
Debt, effective interest rate (percent)																																														4.14%
Actual debt service coverage ratio								1.34	[1]							1.73	[1]		1.53	[1]
Assets	7,520	8,323												2,428	4,190																															467
Total Liabilities	$ 6,788	$ 6,659												$ 762	$ 921																															$ 367

[1] The Debt Service Coverage Ratio is typically calculated over a 12-month historical period and is individually defined for each borrowing in the applicable financing agreement, credit agreement, trust indenture, or other document governing the financing requirements.