Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 2,355		$ 1,948		$ 5,134		$ 5,036
Commodity Margin	820		974		2,057		2,166
Add: Mark-to-market commodity activity, net and other	63	[1]	(99)	[1]	(63)	[2]	21	[2]
Plant operating expense	215		200		741		732
Depreciation and amortization expense	161		166		503		484
Sales, general and other administrative expense	33		33		106		100
Other operating expenses	18		16		55		56
(Income) loss from unconsolidated investments in power plants	(6)		(6)		(16)		(18)
Income from operations	462		466		605		833
Interest expense, net of interest income	157		158		469		468
Debt Extinguishment Costs and Other (Income) Expense, Net	8		1		36		40
Income before income taxes	297		307		100		325
Lease levelization	40		41		(2)		(1)
Amortization of Intangible Assets	25		4		79		11
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	525		737		1,163		1,675
Commodity Margin	298		385		749		843
Add: Mark-to-market commodity activity, net and other	11	[1]	68	[1]	(5)	[2]	173	[2]
Plant operating expense	79		87		268		313
Depreciation and amortization expense	56		61		181		193
Sales, general and other administrative expense	9		7		27		23
Other operating expenses	8		8		23		28
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	157		290		245		459
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,070		713		2,139		1,872
Commodity Margin	198		264		511		583
Add: Mark-to-market commodity activity, net and other	110	[1]	(98)	[1]	7	[2]	(47)	[2]
Plant operating expense	65		62		236		233
Depreciation and amortization expense	53		58		159		157
Sales, general and other administrative expense	13		15		43		47
Other operating expenses	2		2		6		6
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	175		29		74		93
East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	766		506		1,855		1,511
Commodity Margin	324		325		797		740
Add: Mark-to-market commodity activity, net and other	(51)	[1]	(62)	[1]	(44)	[2]	(84)	[2]
Plant operating expense	78		57		258		206
Depreciation and amortization expense	52		48		163		135
Sales, general and other administrative expense	12		10		36		29
Other operating expenses	7		8		27		24
(Income) loss from unconsolidated investments in power plants	(6)		(6)		(16)		(18)
Income from operations	130		146		285		280
Consolidation, Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	(6)		(8)		(23)		(22)
Commodity Margin	0		0		0		0
Add: Mark-to-market commodity activity, net and other	(7)	[1]	(7)	[1]	(21)	[2]	(21)	[2]
Plant operating expense	(7)		(6)		(21)		(20)
Depreciation and amortization expense	0		(1)		0		(1)
Sales, general and other administrative expense	(1)		1		0		1
Other operating expenses	1		(2)		(1)		(2)
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	0		1		1		1
Operating Segments [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2,355		1,948		5,134		5,036
Operating Segments [Member] | West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	524		736		1,159		1,672
Operating Segments [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,067		709		2,129		1,860
Operating Segments [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	764		503		1,846		1,504
Operating Segments [Member] | Consolidation, Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	0		0		0		0
Intersegment Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	0		0		0		0
Intersegment Eliminations [Member] | West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1		1		4		3
Intersegment Eliminations [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	3		4		10		12
Intersegment Eliminations [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2		3		9		7
Intersegment Eliminations [Member] | Consolidation, Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ (6)		$ (8)		$ (23)		$ (22)

[1] Includes $40 million and $41 million of lease levelization and $25 million and $4 million of amortization expense for the three months ended September 30, 2016 and 2015, respectively.

[2] Includes $(2) million and $(1) million of lease levelization and $79 million and $11 million of amortization expense for the nine months ended September 30, 2016 and 2015, respectively.