Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Jun. 30, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 1,646		$ 1,965
Commodity Margin	535	[1]	645	[1]
Add: Mark-to-market commodity activity, net and other	101	[2]	(37)	[2]
Plant operating expense	260		265
Depreciation and amortization expense	158		153
Sales, general and other administrative expense	37		33
Other operating expenses	20		22
(Income) loss from unconsolidated investments in power plants	(5)		(9)
Income from operations	166		144
Interest expense, net of interest income	153		165
Debt extinguishment costs and other (income) expense, net	21		11
Loss before income taxes	(8)		(32)
Lease levelization	(24)		(29)
Contract amortization	4		4
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	517		493
Commodity Margin	218	[1]	202	[1]
Add: Mark-to-market commodity activity, net and other	119	[2]	29	[2]
Plant operating expense	106		105
Depreciation and amortization expense	67		60
Sales, general and other administrative expense	10		10
Other operating expenses	10		12
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	144		44
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	584		659
Commodity Margin	149	[1]	121	[1]
Add: Mark-to-market commodity activity, net and other	41	[2]	(46)	[2]
Plant operating expense	89		90
Depreciation and amortization expense	49		42
Sales, general and other administrative expense	17		12
Other operating expenses	2		2
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	33		(71)
East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	552		844
Commodity Margin	168	[1]	322	[1]
Add: Mark-to-market commodity activity, net and other	(52)	[2]	(11)	[2]
Plant operating expense	72		79
Depreciation and amortization expense	42		51
Sales, general and other administrative expense	10		12
Other operating expenses	8		7
(Income) loss from unconsolidated investments in power plants	(5)		(9)
Income from operations	(11)		171
Geography Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	(7)		(31)
Commodity Margin	0	[1]	0	[1]
Add: Mark-to-market commodity activity, net and other	(7)	[2]	(9)	[2]
Plant operating expense	(7)		(9)
Depreciation and amortization expense	0		0
Sales, general and other administrative expense	0		(1)
Other operating expenses	0		1
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	0		0
Six Power Plants [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Commodity Margin			39	[1]
Operating Segments [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	1,646		1,965
Operating Segments [Member] | West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	515		491
Operating Segments [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	581		647
Operating Segments [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	550		827
Operating Segments [Member] | Geography Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	0		0
Intersegment Eliminations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	0		0
Intersegment Eliminations [Member] | West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2		2
Intersegment Eliminations [Member] | Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	3		12
Intersegment Eliminations [Member] | East [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	2		17
Intersegment Eliminations [Member] | Geography Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ (7)		$ (31)
Six Power Plants [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of power plants disposed					6

[1] 1)Includes $(24) million and $(29) million of lease levelization and $4 million and $4 million of amortization expense for the three months ended March 31, 2015 and 2014, respectively.

[2] (2)Commodity Margin related to the six power plants sold in our East segment on July 3, 2014, was $39 million for the three months ended March 31, 2014.