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Goodwill and Other Intangibles - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2006 USD ($)
Goodwill and Other Intangibles
Goodwill	$ 539,000,000	$ 539,000,000	$ 662,000,000
Goodwill deductible for U.S. income tax purposes	460,000,000	460,000,000	547,000,000
Impairment		26,000,000	64,000,000
Emission allowances held-for-sale	9,000,000	9,000,000
Out-of-market contracts, net	207,000,000	207,000,000	230,000,000
Accumulated amortization, out of market contracts	358,000,000	358,000,000	457,000,000
Emission Allowances
Goodwill and Other Intangibles
Impairment		20,000,000	23,000,000
PPA
Goodwill and Other Intangibles
Impairment		6,000,000	$ 0
Impairment related to power purchase agreements		6,000,000
Texas Genco
Goodwill and Other Intangibles
Goodwill in connection with acquisition					$ 1,700,000,000
EME Project Financings | Lease Agreements
Goodwill and Other Intangibles
Out-of-market contracts, net	159,000,000	159,000,000
SPP
Goodwill and Other Intangibles
Goodwill	0	0
Goodwill, Impairment Loss	12,000,000
BETM | Held-for-sale
Goodwill and Other Intangibles
Goodwill	21,000,000	21,000,000
Goodwill, Impairment Loss	$ 90,000,000
Texas
Goodwill and Other Intangibles
Goodwill, Impairment Loss		$ 337,000,000		$ 1,400,000,000
Percentage of carrying amount	0.43	0.43
EME Project Financings
Goodwill and Other Intangibles
Goodwill	$ 165,000,000	$ 165,000,000
Out-of-market contracts, net				$ 159,000,000
Retail
Goodwill and Other Intangibles
Goodwill	341,000,000	341,000,000
Other
Goodwill and Other Intangibles
Goodwill	33,000,000	33,000,000
Texas
Goodwill and Other Intangibles
Goodwill	$ 0	$ 0

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The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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