Variable Interest Entities and Unconsolidated Investments (VIE Texuals) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 MW	Mar. 31, 2013	Dec. 31, 2013 MW
Variable Interest Entity [Line Items]
Power generation capacity	9,849		9,427
Variable Interest Entity, Financial or Other Support, Amount	$ 40	$ 0
Equity Method Investment, Summarized Financial Information, Debt	376		395
Prorata Share of Equity Method Investment, Summarized Financial Information, Debt	188		198
Greenfield [Member]
Variable Interest Entity [Line Items]
Power generation capacity	1,038
Equity Method Investment, Ownership Percentage	50.00%
Distribution from equity method investee	0	0
Whitby [Member]
Variable Interest Entity [Line Items]
Power generation capacity	50
Equity Method Investment, Ownership Percentage	50.00%
Distribution from equity method investee	$ 13	$ 0
Inland Empire Energy Center [Member]
Variable Interest Entity [Line Items]
Power generation capacity	775
Put Option Exercise Period	2,025
Inland Empire Energy Center [Member] | Minimum [Member]
Variable Interest Entity [Line Items]
Call Option Exercise Period	2,017
Inland Empire Energy Center [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Call Option Exercise Period	2,024