Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restricted Cash and Cash Equivalents Items [Line Items]
Prior Period Reclassification Adjustment		$ 10	$ 9
Held-to-maturity Securities, Restricted	25	25
Current	193	139
Non-current	60	55
Total	253	194
Prior Period Adjustment [Abstract]
Cash and cash equivalents subject to project finance facilities and lease agreements	131	306
Inventory	301	294
Property, plant and equipment, salvage value (as a percent)	10.00%
Property, plant and equipment, salvage value of rotables (as a percent)	0.15%
Impairment of long-lived assets held for use			95
Impairment loss	0	0	116
Asset retirement obligations	38	27
Freestone Energy Center [Member]
Jointly Owned Plants [Abstract]
Jointly Owned Utility Plant, Proportionate Ownership Share	75.00%
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	392
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(124)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	1
Hidalgo Energy Center [Member]
Jointly Owned Plants [Abstract]
Jointly Owned Utility Plant, Proportionate Ownership Share	78.50%
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	252
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(86)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Debt Service
Restricted Cash and Cash Equivalents Items [Line Items]
Current	11	11
Non-current	41	42
Total	52	53
Construction Major Maintenance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	32	33
Non-current	14	10
Total	46	43
Security Project Insurance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	101	79
Non-current	3	0
Total	104	79
Other
Restricted Cash and Cash Equivalents Items [Line Items]
Current	49	16
Non-current	2	3
Total	51	19
Greenfield [Member]
Prior Period Adjustment [Abstract]
Ownership percentage in equity method investment	50.00%
Whitby [Member]
Prior Period Adjustment [Abstract]
Ownership percentage in equity method investment	50.00%
Two Development Projects [Member]
Prior Period Adjustment [Abstract]
Impairment loss			$ 21