Variable Interest Entities and Unconsolidated Investments in Power Plants Variable Interest Entities and Unconsolidated Investments in Power Plants Condensed Financial Statements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,164	$ 1,442	$ 2,779	$ 3,088
Operating expenses	1,027	1,248	2,646	2,733
Income from operations	140	201	143	367
Interest expense, net of interest income	(156)	(157)	(312)	(310)
Other (income) expense, net	(7)	(5)	(13)	(7)
Net income (loss)	$ (24)	$ 21	(218)	14
Greenfield and Whitby [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues			75	98
Operating expenses			43	66
Income from operations			32	32
Interest expense, net of interest income			8	10
Other (income) expense, net			3	(1)
Net income (loss)			$ 21	$ 23