Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 1,164		$ 1,442		$ 2,779		$ 3,088
Commodity Margin	657		657		1,237		1,192
Add: Mark-to-market commodity activity, net and other	(35)	[1]	19	[1]	(126)	[2]	120	[2]
Plant operating expense	271		272		526		532
Depreciation and amortization expense	162		160		342		318
Sales, general and other administrative expense	35		30		73		67
Other operating expenses	17		20		37		40
(Income) loss from unconsolidated investments in power plants	(3)		(7)		(10)		(12)
Income from operations	140		201		143		367
Interest expense, net of interest income	156		157		312		310
Debt Extinguishment Costs and Other (Income) Expense, Net	22		18		28		39
Income (loss) before income taxes	(38)		26		(197)		18
Lease levelization	(20)		(18)		(42)		(42)
Amortization of Intangible Assets	27		3		54		7
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	212		421		638		938
Commodity Margin	254		240		451		458
Add: Mark-to-market commodity activity, net and other	(62)	[1]	(14)	[1]	(16)	[2]	105	[2]
Plant operating expense	98		120		189		226
Depreciation and amortization expense	56		65		125		132
Sales, general and other administrative expense	8		6		18		16
Other operating expenses	7		10		15		20
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	23		25		88		169
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	534		575		1,069		1,159
Commodity Margin	160		170		313		319
Add: Mark-to-market commodity activity, net and other	7	[1]	10	[1]	(103)	[2]	51	[2]
Plant operating expense	85		82		171		171
Depreciation and amortization expense	53		50		106		99
Sales, general and other administrative expense	14		15		30		32
Other operating expenses	2		2		4		4
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	13		31		(101)		64
East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	427		453		1,089		1,005
Commodity Margin	243		247		473		415
Add: Mark-to-market commodity activity, net and other	28	[1]	30	[1]	7	[2]	(22)	[2]
Plant operating expense	96		77		180		149
Depreciation and amortization expense	53		45		111		87
Sales, general and other administrative expense	12		9		24		19
Other operating expenses	10		8		20		16
(Income) loss from unconsolidated investments in power plants	(3)		(7)		(10)		(12)
Income from operations	103		145		155		134
Consolidation, Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	(9)		(7)		(17)		(14)
Commodity Margin	0		0		0		0
Add: Mark-to-market commodity activity, net and other	(8)	[1]	(7)	[1]	(14)	[2]	(14)	[2]
Plant operating expense	(8)		(7)		(14)		(14)
Depreciation and amortization expense	0		0		0		0
Sales, general and other administrative expense	1		0		1		0
Other operating expenses	(2)		0		(2)		0
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	1		0		1		0
Operating Segments [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,164		1,442		2,779		3,088
Operating Segments [Member] | West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	211		421		635		936
Operating Segments [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	530		570		1,062		1,151
Operating Segments [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	423		451		1,082		1,001
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		0		3		2
Intersegment Eliminations [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	4		5		7		8
Intersegment Eliminations [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	4		2		7		4
Intersegment Eliminations [Member] | Consolidation, Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ (9)		$ (7)		$ (17)		$ (14)

[1] Includes $(20) million and $(18) million of lease levelization and $27 million and $3 million of amortization expense for the three months ended June 30, 2016 and 2015, respectively.

[2] Includes $(42) million and $(42) million of lease levelization and $54 million and $7 million of amortization expense for the six months ended June 30, 2016 and 2015, respectively.