Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Current	$ 216	$ 195
Non-current	12	49
Total	$ 228	244
Basis Of Presentation and Summary of Significant Accounting Policies (Textuals) [Abstract]
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 losses	$ 0	123	$ 16
Asset retirement obligations	$ 47	47
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	$ 393
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(148)
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	$ 256
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	(110)
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Debt Service
Restricted Cash and Cash Equivalents Items [Line Items]
Current	28	10
Non-current	8	25
Total	36	35
Rent Reserve [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Current	0	4
Non-current	0	0
Total	0	4
Construction Major Maintenance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	50	54
Non-current	2	17
Total	52	71
Security Project Insurance
Restricted Cash and Cash Equivalents Items [Line Items]
Current	136	127
Non-current	0	5
Total	136	132
Other
Restricted Cash and Cash Equivalents Items [Line Items]
Current	2	0
Non-current	2	2
Total	$ 4	$ 2
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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Jointly Owned Plants [Abstract]

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Property, Plant and Equipment, Salvage Value, Percentage Rotables

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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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The period end amount of construction work in progress at a jointly owned electricity generation plant.

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Cash and equivalents whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements.

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