Fair Value (Nonrecurring Measurements) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Retirement Obligation	$ 80		$ 80		$ 23
Asset Impairment Charges			116	$ 172
Other Asset Impairment Charges	0	$ 74	116	166
Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	114		114
Shady Point [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		55		55		$ 127
Long Lived Assets Held For Sale [Member] | Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges			115
Long Lived Assets Held For Sale [Member] | Fair Value [Member] | Kilroot and Ballylumford [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	0		0
Long Lived Assets Held For Sale [Member] | Fair Value [Member] | Kilroot and Ballylumford [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	118		118
Long Lived Assets Held For Sale [Member] | Fair Value [Member] | Kilroot and Ballylumford [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	0		0
Long Lived Assets Held For Sale [Member] | Carrying Amount [Member] | Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	$ 232		$ 232
Long Lived Assets Held And Used [Member] | Shady Point [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges				156
Long Lived Assets Held And Used [Member] | Fair Value [Member] | Shady Point [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		0
Long Lived Assets Held And Used [Member] | Fair Value [Member] | Shady Point [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		0
Long Lived Assets Held And Used [Member] | Fair Value [Member] | Shady Point [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		33		33
Long Lived Assets Held And Used [Member] | Carrying Amount [Member] | Shady Point [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		185		185
Equity Method Investments [Member] | Elsta [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges				5
Equity Method Investments [Member] | Fair Value [Member] | Elsta [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		0
Equity Method Investments [Member] | Fair Value [Member] | Elsta [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		16		16
Equity Method Investments [Member] | Fair Value [Member] | Elsta [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		0
Equity Method Investments [Member] | Carrying Amount [Member] | Elsta [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		$ 21		$ 21
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring			4.00%
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring			4.38%
Interest Rate Contract [Member] | Measurement Input, Entity Credit Risk [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring			2.00%