Variable Interest Entities and Unconsolidated Investments in Power Plants (VIE Texuals) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014 MW	Jun. 30, 2013	Jun. 30, 2014 MW	Jun. 30, 2013	Dec. 31, 2013 MW
Variable Interest Entity [Line Items]
Power generation capacity	10,365		10,365		9,427
Variable Interest Entity, Financial or Other Support, Amount	$ 0	$ 0	$ 40	$ 0
Equity Method Investment, Summarized Financial Information, Debt	385		385		395
Prorata Share of Equity Method Investment, Summarized Financial Information, Debt	193		193		198
Greenfield [Member]
Variable Interest Entity [Line Items]
Power generation capacity	1,038		1,038
Equity Method Investment, Ownership Percentage	50.00%		50.00%
Distribution from equity method investee	0	7	0	7
Whitby [Member]
Variable Interest Entity [Line Items]
Power generation capacity	50		50
Equity Method Investment, Ownership Percentage	50.00%		50.00%
Distribution from equity method investee	$ 0	$ 9	$ 13	$ 9
Inland Empire Energy Center [Member]
Variable Interest Entity [Line Items]
Power generation capacity	775		775
Put Option Exercise Period	2,025		2,025
Inland Empire Energy Center [Member] | Minimum [Member]
Variable Interest Entity [Line Items]
Call Option Exercise Period	2,017		2,017
Inland Empire Energy Center [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Call Option Exercise Period	2,024		2,024