Debt and Credit Agreements (Project Financings) (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended	3 Months Ended					1 Months Ended			12 Months Ended												1 Months Ended				1 Months Ended							12 Months Ended		12 Months Ended		12 Months Ended				12 Months Ended			12 Months Ended				12 Months Ended	1 Months Ended	12 Months Ended								12 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Mar. 30, 2012 Broken Bow and Crofton Bluffs Wind Projects	Mar. 31, 2012 Broken Bow and Crofton Bluffs Wind Projects	Dec. 31, 2013 Broken Bow Wind, LLC		Dec. 31, 2013 Crofton Bluffs		Dec. 31, 2011 Tapestry Wind, LLC	Dec. 31, 2013 Tapestry Wind, LLC		Dec. 31, 2013 Walnut Creek	Jul. 31, 2011 Walnut Creek	Dec. 31, 2013 Laredo Ridge		Dec. 31, 2013 Cedro Hill Wind LLC [Member]		Oct. 31, 2009 Big Sky Wind, LLC MW	Dec. 31, 2013 High Lonesome Mesa, LLC		Dec. 31, 2013 Viento Funding II, Inc.		Jul. 31, 2013 Viento Funding II, Inc.	Dec. 31, 2011 Working Capital Facility Tapestry Wind, LLC	Jul. 31, 2013 Working Capital Facility Viento Funding II, Inc.	Dec. 31, 2013 Letter of Credit Tapestry Wind, LLC	Dec. 31, 2013 Letter of Credit High Lonesome Mesa, LLC	Jul. 31, 2013 Letter of Credit Viento Funding II, Inc.	Dec. 31, 2011 Term Loan Tapestry Wind, LLC	Jul. 31, 2013 Term Loan Viento Funding II, Inc.	Dec. 31, 2011 Debt Service Reserve Letter of Credit Tapestry Wind, LLC	Dec. 31, 2011 Project Letter of Credit Tapestry Wind, LLC	Dec. 31, 2013 Others	Dec. 31, 2012 Others	Dec. 31, 2013 Term Loan Due 2027 Broken Bow Wind, LLC	Dec. 31, 2012 Term Loan Due 2027 Broken Bow Wind, LLC	Dec. 31, 2013 Term Loan Due 2027 Crofton Bluffs	Dec. 31, 2012 Term Loan Due 2027 Crofton Bluffs	Dec. 31, 2013 Term Loan due 2021 Tapestry Wind, LLC	Dec. 31, 2012 Term Loan due 2021 Tapestry Wind, LLC	Dec. 31, 2013 Term Loan Due 2023 Viento Funding II, Inc.	Dec. 31, 2012 Term Loan Due 2023 Viento Funding II, Inc.	Dec. 31, 2013 Term Loan Due 2023 Construction Loan Walnut Creek	Jun. 01, 2013 Term Loan Due 2023 Construction Loan Walnut Creek	Dec. 31, 2012 Term Loan Due 2023 Construction Loan Walnut Creek	Dec. 31, 2013 Term Loan Due 2023 Construction Loan WCEP Holdings, LLC	Dec. 31, 2012 Term Loan Due 2023 Construction Loan WCEP Holdings, LLC	Dec. 31, 2013 Construction Loan due 2013 [Member] Construction Loan Walnut Creek	Dec. 31, 2013 Construction Loan due 2013 [Member] Construction Loan WCEP Holdings, LLC	Dec. 31, 2013 Term Loan due 2020 Viento Funding II, Inc.	Oct. 31, 2009 Vendor financing loan due 2014 Big Sky Wind, LLC	Dec. 31, 2013 Vendor financing loan due 2014 Big Sky Wind, LLC	Dec. 31, 2012 Vendor financing loan due 2014 Big Sky Wind, LLC	Dec. 31, 2013 Letter of Credit	Dec. 31, 2013 Letter of Credit Walnut Creek	Dec. 31, 2013 Bonds due 2017 High Lonesome Mesa, LLC	Dec. 31, 2012 Bonds due 2017 High Lonesome Mesa, LLC	Dec. 31, 2013 Bonds due 2017 American Bituminous	Dec. 31, 2012 Bonds due 2017 American Bituminous	Dec. 31, 2013 Term Loan due 2025 Cedro Hill Wind LLC [Member]	Dec. 31, 2012 Term Loan due 2025 Cedro Hill Wind LLC [Member]	Dec. 31, 2013 Term Loan due 2026 Laredo Ridge	Dec. 31, 2012 Term Loan due 2026 Laredo Ridge
Debt Instrument [Line Items]
Date of issuance			March 30, 2012						December 2011				July 2011										July 2013																												October 2009
Construction loans			$ 79
Term of amortizing loan (in years)				15 years																				10 years					10 years		10 years	10 years
Letter of credit facilities			13																					8							12	8
Working capital facilities			6
Floating interest rate received			LIBOR						LIBOR																					LIBOR					LIBOR		LIBOR		LIBOR				LIBOR			LIBOR				LIBOR		LIBOR								LIBOR		LIBOR
Variable rate, spread (as a percent)			2.875%						2.50%																					2.75%					2.88%		2.88%		2.50%									2.25%	4.00%	2.75%		3.50%								3.13%		2.75%
Current interest rate (percent)																																			3.12%		3.12%		2.75%									2.50%	4.24%	3.10%		3.87%								3.38%		3.00%
Debt Instrument, Interest Rate, Stated Percentage																																																								6.85%		0.05%
Variable rate, increase rate after every three years (as a percent)			0.125%
Debt	1,498	1,438																															29	43	50	52	26	27	201	210	200	191	417	495	330	53	52					231	222			65	69	39	46	118	125	69	71
Short-term debt	4	382
Outstanding letters of credit					10		3																			20	7
Long-term Debt, Gross																													214																							231
Rate increase on every fourth anniversary									0.125%																					0.25%
Proceeds from Issuance of Debt																													97
Standby letters of credit																																																						163	22
Debt instrument, face amount																														202
Long-term debt	1,494	1,056
Generating capacity (in megawatts)																		240
Debt maturity period												10 years													7 years			7 years		10 years																					5 years
Debt, effective interest rate (percent)																																			5.56%		5.37%		4.51%		5.61%		5.46%			7.63%						3.87%				6.85%		0.05%		7.01%		5.90%
Actual debt service coverage ratio					1.67	[1]	2.23	[1]		1.48	[1]			1.79	[1]	1.53	[1]		1.88	[1],[2]	2.75	[1]
Assets	6,973	7,520																																																		447
Total Liabilities	6,712	6,788																																																		369
Current portion of long-term debt	323	307
Long-term debt net of current portion	$ 1,171	$ 749

[1] The Debt Service Coverage Ratio (DSCR) 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.

[2] Calculated at October 31, 2013, the last payment date.