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Goodwill and Other Intangibles - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Other Intangibles
Goodwill	$ 573,000,000	$ 573,000,000	$ 539,000,000
Goodwill deductible for U.S. income tax purposes	366,000,000	366,000,000	460,000,000
Goodwill, Impairment Loss				$ 337,000,000	$ 1,400,000,000
Goodwill, Acquired During Period				1,700,000,000
Impairment		6,000,000	20,000,000
Emission allowances held-for-sale	12,000,000	12,000,000	9,000,000
Out-of-market contracts, net	121,000,000	121,000,000	129,000,000
Accumulated amortization, out of market contracts	37,000,000	37,000,000	29,000,000
Emission Allowances
Goodwill and Other Intangibles
Impairment		5,000,000	$ 20,000,000
Impairment related to power purchase agreements		0
Retail
Goodwill and Other Intangibles
Goodwill	408,000,000	408,000,000
Midwest Generation | Lease Agreements
Goodwill and Other Intangibles
Out-of-market contracts, net	121,000,000	121,000,000
BETM | Held-for-sale
Goodwill and Other Intangibles
Goodwill	21,000,000	21,000,000
Goodwill, Impairment Loss	90,000,000
EME Project Financings
Goodwill and Other Intangibles
Goodwill	$ 165,000,000	$ 165,000,000
Out-of-market contracts, net				$ 159,000,000

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Accumulated Amortization of Out of Market Contracts

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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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