Debt and Credit Agreements (Narrative Disclosures for Long-Term Debt) (Details) (USD $) In Millions, unless otherwise specified					1 Months Ended						1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Unsecured Notes	Jul. 31, 2013 Viento Funding II, Inc.	Jul. 31, 2013 Viento Funding II, Inc. Term Loan	Jul. 31, 2013 Viento Funding II, Inc. Working Capital Facility	Jul. 31, 2013 Viento Funding II, Inc. Letter of Credit	Dec. 31, 2013 High Lonesome Mesa, LLC Letter of Credit	Dec. 31, 2013 High Lonesome Mesa, LLC Bonds Series 2010A Bonds	Dec. 31, 2013 High Lonesome Mesa, LLC Bonds Series 2010B Bonds	Dec. 31, 2011 Tapestry Wind, LLC	Dec. 31, 2013 Tapestry Wind, LLC Letter of Credit	Oct. 31, 2009 Big Sky Wind, LLC Vendor financing loan due 2014	Dec. 31, 2013 Big Sky Wind, LLC Vendor financing loan due 2014
Debt Instrument [Line Items]
Debt, effective interest rate (percent)														3.87%
Debt instrument, face amount			$ 3,700		$ 202
Long-term debt maturities, 2013	323
Long-term debt maturities, 2014	93
Long-term debt maturities, 2015	91
Long-term debt maturities, 2016	141
Long-term debt maturities, 2017	91
Date of issuance				July 2013							December 2011		October 2009
Increased financing amount				238
Credit facility						9	27
Debt maturity period					10 years	7 years	7 years						5 years
Rate increase on every fourth anniversary					0.25%						0.125%
Variable rate, spread (as a percent)					2.75%						2.50%			3.50%
Variable rate, basis					LIBOR						LIBOR			LIBOR
Long-term debt	1,494	1,056							40	25
Outstanding letters of credit								$ 7				$ 20