Debt and Credit Agreements (Narrative Disclosures for Long-Term Debt) (Details) (USD $)						1 Months Ended					0 Months Ended																0 Months Ended	1 Months Ended						1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Unsecured Notes	Dec. 31, 2012 Viento Funding II, Inc.	Feb. 28, 2011 Viento Funding II, Inc.	Feb. 28, 2011 Viento Funding II, Inc. Term Loan	Feb. 28, 2011 Viento Funding II, Inc. Working Capital Facility	Feb. 28, 2011 Viento Funding II, Inc. Letter of Credit	Dec. 31, 2012 Viento Funding II, Inc. Letter of Credit	Mar. 31, 2010 Cedro Hill Wind, LLC	Dec. 22, 2010 Cedro Hill Wind, LLC Term Loan	Dec. 31, 2012 Cedro Hill Wind, LLC Term Loan	Mar. 31, 2010 Cedro Hill Wind, LLC Term Loan	Mar. 31, 2010 Cedro Hill Wind, LLC Working Capital Facility	Dec. 31, 2012 Cedro Hill Wind, LLC Letter of Credit	Mar. 31, 2010 Cedro Hill Wind, LLC Letter of Credit	Mar. 31, 2010 Cedro Hill Wind, LLC Construction Loan	Nov. 30, 2010 High Lonesome Mesa, LLC	Dec. 31, 2012 High Lonesome Mesa, LLC Letter of Credit	Nov. 30, 2010 High Lonesome Mesa, LLC Letter of Credit	Jun. 30, 2011 High Lonesome Mesa, LLC Letter of Credit Letter of Credit Reimbursement Agreement	Dec. 31, 2012 High Lonesome Mesa, LLC Bonds Series 2010A Bonds	Nov. 30, 2010 High Lonesome Mesa, LLC Bonds Series 2010A Bonds	Dec. 31, 2012 High Lonesome Mesa, LLC Bonds Series 2010B Bonds	Nov. 30, 2010 High Lonesome Mesa, LLC Bonds Series 2010B Bonds	Jul. 31, 2010 Laredo Ridge	Mar. 18, 2010 Laredo Ridge Term Loan	Mar. 18, 2011 Laredo Ridge Term Loan	Dec. 31, 2012 Laredo Ridge Term Loan	Jul. 31, 2010 Laredo Ridge Working Capital Facility	Dec. 31, 2012 Laredo Ridge Letter of Credit	Jul. 31, 2010 Laredo Ridge Letter of Credit	Jul. 31, 2010 Laredo Ridge Construction Loan	Dec. 31, 2011 Tapestry Wind, LLC	Dec. 31, 2012 Tapestry Wind, LLC Letter of Credit	Oct. 31, 2009 Big Sky Wind, LLC Vendor financing loan due 2014	Dec. 31, 2012 Big Sky Wind, LLC Vendor financing loan due 2014
Debt Instrument [Line Items]
Debt instrument, face amount			$ 3,700,000,000			$ 227,000,000								$ 4,000,000		$ 10,000,000	$ 135,000,000	$ 81,000,000		$ 6,000,000			$ 50,000,000		$ 25,000,000					$ 3,000,000		$ 9,000,000	$ 75,000,000
Nonrecourse debt	1,400,000,000
Long-term debt maturities, 2013	70,000,000
Long-term debt maturities, 2014	297,000,000
Long-term debt maturities, 2015	75,000,000
Long-term debt maturities, 2016	69,000,000
Long-term debt maturities, 2017	112,000,000
Date of issuance					February 2011					March 2010	December 22, 2010							November 2010			June 2011					July 2010		March 18, 2011						December 2011		October 2009
Increased financing amount					255,000,000
Credit facility							5,000,000	23,000,000
Debt maturity period						10 years	7 years	7 years			15 years																	15 years								5 years
Rate increase on every fourth anniversary						0.25%																												0.125%
Variable rate, spread (as a percent)						2.75%							3.00%															2.75%						2.50%			3.50%
Variable rate, basis						LIBOR					LIBOR																LIBOR							LIBOR			LIBOR
Debt consent fee				1,000,000
Long-term debt	1,056,000,000	4,912,000,000		191,000,000								125,000,000			10,000,000				11,000,000		7,000,000	44,000,000		25,000,000					71,000,000		9,000,000
Outstanding letters of credit									$ 23,000,000																										$ 20,000,000