Variable Interest Entities and Unconsolidated Investments in Power Plants Condensed Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating revenues	$ 1,646us-gaap_Revenues	$ 1,965us-gaap_Revenues
Costs and Expenses	1,485us-gaap_CostsAndExpenses	1,830us-gaap_CostsAndExpenses
Net income (loss)	(7)us-gaap_ProfitLoss	(13)us-gaap_ProfitLoss
Whitby [Member]
Operating revenues	15	15
Costs and Expenses	9	7
Net income (loss)	$ 6	$ 8

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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