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Asset Impairments (Details) - USD ($)			1 Months Ended	3 Months Ended						12 Months Ended
	May 12, 2016	Mar. 01, 2016	Nov. 30, 2014	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 14, 2017
Asset Impairments
Construction in progress						$ 0
Impairment losses										$ 1,709,000,000	$ 702,000,000	$ 4,860,000,000
Other impairment losses											48,000,000
Impairment losses on investments									$ 56,000,000	$ 79,000,000	268,000,000	56,000,000
Elbow Creek
Asset Impairments
Impairment losses							$ 117,000,000
Goat Wind
Asset Impairments
Impairment losses							60,000,000
Forward
Asset Impairments
Impairment losses							6,000,000
Long Beach
Asset Impairments
Impairment losses							36,000,000
Other
Asset Impairments
Impairment losses											153,000,000
Emission Allowances
Asset Impairments
Impairment losses											23,000,000
Other Intangible Assets
Asset Impairments
Impairment losses											23,000,000
Solar Panels
Asset Impairments
Impairment losses											19,000,000
Deferred Marketing Expenses
Asset Impairments
Impairment losses											18,000,000
Limestone
Asset Impairments
Impairment losses									1,514,000,000
W.A. Parish
Asset Impairments
Impairment losses									1,295,000,000
Huntley
Asset Impairments
Impairment losses												132,000,000
Term of agreement		4 years
Dunkirk
Asset Impairments
Impairment losses												160,000,000
Term of agreement			10 years
Gregory Power Partners, L.P.
Asset Impairments
Impairment losses												$ 176,000,000
Rockford
Asset Impairments
Impairment losses											17,000,000
Percentage of ownership interest	100.00%
Proceeds from sale	$ 55,000,000
Renewables
Asset Impairments
Impairment losses				$ 15,000,000	$ 14,000,000	22,000,000
Collectibility of Receivables | BTEC New Albany, LLC
Asset Impairments
Loss contingency, receivable													$ 48,000,000
Collectibility of Receivables | BTEC New Albany, LLC | Purchaser Incurred Costs
Asset Impairments
Loss contingency, receivable													38,000,000
Collectibility of Receivables | BTEC New Albany, LLC | Liquidation Damages
Asset Impairments
Loss contingency, receivable													$ 10,000,000
Construction in Progress
Asset Impairments
Impairment losses						$ 41,000,000
South Texas Project
Asset Impairments
Impairment loss investments				1,248,000,000
Indian River
Asset Impairments
Impairment loss investments				36,000,000
Keystone
Asset Impairments
Impairment loss investments				35,000,000
Conemaugh
Asset Impairments
Impairment loss investments				35,000,000
Langford
Asset Impairments
Impairment loss investments				110,000,000
Elbow Creek
Asset Impairments
Impairment loss investments				26,000,000
Forward
Asset Impairments
Impairment loss investments				4,000,000
Petra Nova Parish Holdings
Asset Impairments
Impairment loss investments				$ 69,000,000				$ 140,000,000
Ownership percentage				50.00%				50.00%		50.00%
Other
Asset Impairments
Impairment loss investments				$ 11,000,000							$ 22,000,000
Community Wind North, LLC
Asset Impairments
Impairment loss investments							36,000,000
Sherbino I Wind Farm LLC
Asset Impairments
Impairment loss investments							$ 70,000,000
Solar Panels
Asset Impairments
Impairment losses									$ 29,000,000

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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 amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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