Debt and Credit Agreements (Project Financings) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended						1 Months Ended	9 Months Ended	1 Months Ended				1 Months Ended	9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Walnut Creek Project MW	Jul. 31, 2013 Viento Funding II, Inc.	Oct. 31, 2009 Big Sky Wind, LLC MW	Jul. 31, 2013 Working Capital Facility Viento Funding II, Inc.	Sep. 30, 2013 Term Loan Walnut Creek Project	Jul. 31, 2013 Term Loan Viento Funding II, Inc.	Jul. 31, 2013 Letter of Credit Viento Funding II, Inc.	Sep. 30, 2013 Letter of Credit High Lonesome Mesa, LLC	Sep. 30, 2013 Series 2010A Bonds Bonds High Lonesome Mesa, LLC	Oct. 31, 2009 Vendor financing loan due 2014 Big Sky Wind, LLC	Sep. 30, 2013 Vendor financing loan due 2014 Big Sky Wind, LLC	Sep. 30, 2013 Letter of Credit	Sep. 30, 2013 Letter of Credit Walnut Creek Project	Sep. 30, 2013 Series 2010B Bonds Bonds High Lonesome Mesa, LLC	Sep. 30, 2013 Terminated Letter of Credit	Sep. 30, 2013 Terminated Interest rate swap Viento Funding II, Inc.	Jul. 31, 2013 Terminated Interest rate swap Viento Funding II, Inc.
Debt Instrument [Line Items]
Date of issuance							July 2013								October 2009
Debt Instrument, Financing Amount Under Agreement							$ 238
Debt Instrument, Face Amount											202
Debt Instrument, Description of Variable Rate Basis											LIBOR					LIBOR
Debt Instrument, Basis Spread on Variable Rate											2.75%					3.50%
Debt Instrument, Increase in Basis Spread of Variable Interest Rate on Every Fourth Anniversary											0.25%
Long-term debt outstanding																228
Standby letters of credit																	169	22		0
Generating capacity (in megawatts)						479		240
Debt maturity period									7 years	10 years	10 years	7 years			5 years
Credit facility, maximum borrowing capacity									9			27
Debt, effective interest rate (percent)																3.94%
Assets	7,054		7,054		7,520											451
Total Liabilities	6,795		6,795		6,788											369
Notional value of derivative																						78
Fixed swap rate																						3.415%
(Gain) loss on early extinguishment of debt	(3)	0	(3)	0																	4
Long-term Debt	1,523		1,523		1,056									41					25
Letters of Credit Outstanding, Amount													$ 7