Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Cash and Cash Equivalents Items [Line Items]
Current	$ 195	$ 203
Non-current	49	69
Total	244	272
Basis Of Presentation and Summary of Significant Accounting Policies (Textuals) [Abstract]
Cash and cash equivalents subject to project finance facilities and lease agreements	257	292
Property, plant and equipment, salvage value (as a percent)	10.00%
Property, plant and equipment, salvage value of rotables (as a percent)	0.15%
Osprey Energy Center Agreement Term	0 years 27 months
Impairment losses	123us-gaap_AssetImpairmentCharges	16us-gaap_AssetImpairmentCharges	0us-gaap_AssetImpairmentCharges
Asset retirement obligations	47us-gaap_AssetRetirementObligation	44us-gaap_AssetRetirementObligation
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	389
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(140)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
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	257
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(104)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Debt Service
Restricted Cash and Cash Equivalents Items [Line Items]
Current	10	11
Non-current	25	41
Total	35	52
Rent Reserve [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Current	4	3
Non-current	0	0
Total	4	3
Construction Major Maintenance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	54	35
Non-current	17	20
Total	71	55
Security Project Insurance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	127	151
Non-current	5	6
Total	132	157
Other
Restricted Cash and Cash Equivalents Items [Line Items]
Current	0	3
Non-current	2	2
Total	$ 2	$ 5
Greenfield [Member]
Basis Of Presentation and Summary of Significant Accounting Policies (Textuals) [Abstract]
Ownership percentage in equity method investment	50.00%
Whitby [Member]
Basis Of Presentation and Summary of Significant Accounting Policies (Textuals) [Abstract]
Ownership percentage in equity method investment	50.00%

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The period end amount owned by the reporting utility of a jointly owned utility plant in service.

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