Derivative Instruments and Hedging Activities (Interest Rate Risk Management) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013 Interest rate contracts	Jun. 30, 2013 Interest rate contracts	Jun. 30, 2013 Viento Funding II, Inc. Interest rate contracts Swap 1	Dec. 31, 2012 Viento Funding II, Inc. Interest rate contracts Swap 1	Jun. 30, 2013 Viento Funding II, Inc. Interest rate contracts Swap 2	Dec. 31, 2012 Viento Funding II, Inc. Interest rate contracts Swap 2	Jun. 30, 2013 Cedro Hill Project Interest rate contracts	Dec. 31, 2012 Cedro Hill Project Interest rate contracts	Jun. 30, 2013 Laredo Ridge Interest rate contracts	Dec. 31, 2012 Laredo Ridge Interest rate contracts	Jun. 30, 2013 WCEP Holdings, LLC Interest rate contracts		Dec. 31, 2012 WCEP Holdings, LLC Interest rate contracts		Jun. 30, 2013 WCEP Holdings, LLC Forward Starting Interest Rate Contracts		Dec. 31, 2012 WCEP Holdings, LLC Forward Starting Interest Rate Contracts		Jun. 30, 2013 Walnut Creek Energy Interest rate contracts		Dec. 31, 2012 Walnut Creek Energy Interest rate contracts		Jun. 30, 2013 Walnut Creek Energy Forward Starting Interest Rate Contracts		Dec. 31, 2012 Walnut Creek Energy Forward Starting Interest Rate Contracts		Jun. 30, 2013 Tapestry Wind, LLC Interest rate contracts	Dec. 31, 2012 Tapestry Wind, LLC Interest rate contracts	Jun. 30, 2013 Tapestry Wind, LLC Forward Starting Interest Rate Contracts	Dec. 31, 2012 Tapestry Wind, LLC Forward Starting Interest Rate Contracts	Jun. 30, 2013 Broken Bow Wind, LLC Interest rate contracts	Dec. 31, 2012 Broken Bow Wind, LLC Interest rate contracts	Jun. 30, 2013 Broken Bow Wind, LLC Forward Starting Interest Rate Contracts	Dec. 31, 2012 Broken Bow Wind, LLC Forward Starting Interest Rate Contracts	Jun. 30, 2013 Crofton Bluffs Interest rate contracts	Dec. 31, 2012 Crofton Bluffs Interest rate contracts	Jun. 30, 2013 Crofton Bluffs Forward Starting Interest Rate Contracts	Dec. 31, 2012 Crofton Bluffs Forward Starting Interest Rate Contracts
Derivative [Line Items]
Fixed swap rate paid			3.18%		3.42%		4.29%		3.46%		0.79%	[1]			4.00%	[1]			0.81%	[1]			3.54%	[1]			2.21%		3.57%		0.83%		2.96%		0.78%		2.75%
Notional value			$ 55	$ 65	$ 108	$ 108	$ 108	$ 112	$ 63	$ 64	$ 0	[1]	$ 26	[1]	$ 48	[1]	$ 0	[1]	$ 0	[1]	$ 181	[1]	$ 398	[1]	$ 0	[1]	$ 184	$ 189	$ 60	$ 60	$ 46	$ 47	$ 45	$ 45	$ 24	$ 24	$ 23	$ 23
Unrealized gains on interest rate swaps recorded in earnings	$ 4	$ 5

[1] During the second quarter of 2013, the existing interest rate swaps for the Walnut Creek Project expired and, in conjunction with the conversion to term loans, the forward starting swaps became effective.