Fair Value (Nonrecurring Measurements) (Details) - USD ($)		3 Months Ended					12 Months Ended
		Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 01, 2018
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges		$ 69,000,000	$ 116,000,000	$ 42,000,000	$ 74,000,000	$ 92,000,000	$ 185,000,000	$ 208,000,000	$ 537,000,000
Other non-operating expense							92,000,000	147,000,000	0
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax									$ (611,000,000)
Shady Point [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring				30,000,000				30,000,000
Nejapa [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring				5,000,000				5,000,000
Guacolda Affiliate [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Equity Method Investment, Other than Temporary Impairment								144,000,000
Kilroot and Ballylumford [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		114,000,000					114,000,000
OPGC Affiliate [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Equity Method Investment, Other than Temporary Impairment		92,000,000
Estimate of Fair Value Measurement [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		315,000,000					315,000,000
Equity Method Investments [Member] | Elsta [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges								3,000,000
Equity Method Investments [Member] | Estimate of Fair Value Measurement [Member] | Elsta [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring					0
Equity Method Investments [Member] | Estimate of Fair Value Measurement [Member] | Elsta [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring					16,000,000
Equity Method Investments [Member] | Estimate of Fair Value Measurement [Member] | Elsta [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring					0
Equity Method Investments [Member] | Reported Value Measurement [Member] | Elsta [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1]				19,000,000
Equity Method Investments [Member] | Reported Value Measurement [Member] | Guacolda Affiliate [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1]									$ 354,000,000
Long Lived Assets Held And Used [Member] | Shady Point [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges	[2]							157,000,000
Long Lived Assets Held And Used [Member] | Nejapa [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges	[3]							37,000,000
Long Lived Assets Held And Used [Member] | Kilroot [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		20,000,000					20,000,000
Long Lived Assets Held And Used [Member] | US Generation Facility [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges	[3]						60,000,000
Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Nejapa [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]				0
Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Nejapa [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]				$ 0
Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Nejapa [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]			5,000,000				5,000,000
Long Lived Assets Held And Used [Member] | Reported Value Measurement [Member] | Nejapa [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1],[3]			42,000,000				42,000,000
Long Lived Assets Held For Sale [Member] | Guacolda Affiliate [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring				0				0
Long Lived Assets Held For Sale [Member] | Guacolda Affiliate [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring				0				0
Long Lived Assets Held For Sale [Member] | Guacolda Affiliate [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring				209,000,000				209,000,000
Long Lived Assets Held For Sale [Member] | Kilroot and Ballylumford [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Other Asset Impairment Charges	[2]						115,000,000
Assets Carrying Amount Disclosure Nonrecurring			232,000,000
Long Lived Assets Held For Sale [Member] | OPGC Affiliate [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		212,000,000					212,000,000
Long Lived Assets Held For Sale [Member] | Estimate of Fair Value Measurement [Member] | Kilroot and Ballylumford [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]		0
Long Lived Assets Held For Sale [Member] | Estimate of Fair Value Measurement [Member] | Kilroot and Ballylumford [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]		118,000,000
Long Lived Assets Held For Sale [Member] | Estimate of Fair Value Measurement [Member] | Kilroot and Ballylumford [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]		$ 0
Long Lived Assets Held For Sale [Member] | Estimate of Fair Value Measurement [Member] | OPGC Affiliate [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0					0
Long Lived Assets Held For Sale [Member] | Estimate of Fair Value Measurement [Member] | OPGC Affiliate [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0					0
Long Lived Assets Held For Sale [Member] | Reported Value Measurement [Member] | OPGC Affiliate [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1]	304,000,000					304,000,000
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Shady Point [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]			0				0
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Shady Point [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]	0					0
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | Shady Point [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]	30,000,000					30,000,000
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | US Generation Facility [Member] | Level 1
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]	0					0
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | US Generation Facility [Member] | Level 2
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]	0					0
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Estimate of Fair Value Measurement [Member] | US Generation Facility [Member] | Level 3
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[3]	103,000,000					103,000,000
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Reported Value Measurement [Member] | Shady Point [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1],[2]			$ 211,000,000				$ 211,000,000
Fair Value, Nonrecurring [Member] | Long Lived Assets Held And Used [Member] | Reported Value Measurement [Member] | US Generation Facility [Member]
Fair Value Assets Measured On Nonrecurring Basis Unobservable Inputs [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[1],[3]	$ 163,000,000					$ 163,000,000

[1] Represents the carrying values at the dates of initial measurement, before fair value adjustment.

[2] Per the Company's policy, pre-tax loss is limited to the impairment of long-lived assets. Any additional loss will be recognized on completion of the sale. See Note 22—Asset Impairment Expense and Note 25—Held-for-Sale and Dispositions for further information.

[3] See Note 22—Asset Impairment Expense for further information.