Debt and Credit Agreements (Schedule of Debt) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Total debt	$ 1,522	$ 1,438
Less: Short-term debt	0	382
Long-term debt	1,522	1,056
Less: Current portion of long-term debt	118	307
Long-term debt net of current portion	1,404	749
Unsecured Notes
Debt Instrument [Line Items]
Debt Instrument, Face Amount		3,700
Other Debt Types
Debt Instrument [Line Items]
Total debt	38	43
WCEP Holdings, LLC | Construction Loans | Term Loan due 2023 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	7.63%
Total debt	53	52
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	4.00%
Debt Instrument, Interest Rate at Period End	4.32%
Big Sky Wind, LLC | Vendor financing loan due 2014
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	3.94%
Total debt	226	222
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	3.50%
Debt Instrument, Interest Rate at Period End	3.94%
High Lonesome Mesa, LLC | Bonds | Bonds due 2017
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.85%
Debt, effective interest rate (percent)	6.85%
Total debt	66	69
American Bituminous Power Partners, L.P. [Member] | Bonds | Bonds due 2017
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.14%
Debt, effective interest rate (percent)	0.14%
Total debt	46	46
Viento Funding II, Inc. | Term Loan | Term Loan due 2020
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	5.82%
Total debt	179	191
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.75%
Debt Instrument, Interest Rate at Period End	3.18%
Tapestry Wind, LLC | Term Loan | Term Loan Due 2021 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	4.52%
Total debt	205	210
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.50%
Debt Instrument, Interest Rate at Period End	2.79%
Cedro Hill Project | Term Loan | Term Loan Due 2025 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	6.89%
Total debt	120	125
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	3.00%
Debt Instrument, Interest Rate at Period End	3.29%
Laredo Ridge | Term Loan | Term Loan due 2026 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	5.90%
Total debt	70	71
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.75%
Debt Instrument, Interest Rate at Period End	3.03%
Crofton Bluffs Project | Term Loan | Term Loan Due 2027 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	3.61%
Total debt	26	27
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.88%
Debt Instrument, Interest Rate at Period End	3.16%
Broken Bow I Project | Term Loan | Term Loan Due 2027 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	3.65%
Total debt	51	52
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.88%
Debt Instrument, Interest Rate at Period End	3.15%
Walnut Creek Energy | Construction Loans | Term Loan due 2023 [Member]
Debt Instrument [Line Items]
Debt, effective interest rate (percent)	5.47%
Total debt	$ 442	$ 330
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	2.25%
Debt Instrument, Interest Rate at Period End	2.53%