Fair Value (Nonrecurring Measurements) (Details) - USD ($)		3 Months Ended					6 Months Ended
		Jun. 30, 2020		Mar. 31, 2020		Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Apr. 12, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges		$ 0				$ 116,000,000	$ 6,000,000		$ 116,000,000
Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring						$ 114,000,000			114,000,000
Long Lived Assets Held And Used [Member] | Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges	[1]								$ 115,000,000
Long Lived Assets Held And Used [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Asset Impairment Charges				$ 43,000,000			158,000,000
Long Lived Assets Held And Used [Member] | Fair Value [Member] | Kilroot and Ballylumford [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]									$ 232,000,000
Long Lived Assets Held And Used [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
Long Lived Assets Held And Used [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										118,000,000
Long Lived Assets Held And Used [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
Long Lived Assets Held And Used [Member] | Fair Value [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring	[2]	272,000,000	[3]	195,000,000	[4]		272,000,000	[3]
Cumulative Translation Adjustment, Net of Tax, Period Increase (Decrease)				115,000,000			114,000,000
Long Lived Assets Held And Used [Member] | Fair Value [Member] | OPGC [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		0			0
Long Lived Assets Held And Used [Member] | Fair Value [Member] | OPGC [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		104,000,000		0			104,000,000
Long Lived Assets Held And Used [Member] | Fair Value [Member] | OPGC [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		0		$ 152,000,000			$ 0
Measurement Input, Long-term Revenue Growth Rate [Member] | Valuation, Income Approach [Member] | Long Lived Assets Held And Used [Member] | Weighted Average [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring							2.00%
Measurement Input, Long-term Revenue Growth Rate [Member] | Valuation, Income Approach [Member] | Long Lived Assets Held And Used [Member] | Maximum [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring							40.00%
Measurement Input, Long-term Revenue Growth Rate [Member] | Valuation, Income Approach [Member] | Long Lived Assets Held And Used [Member] | Minimum [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Inputs, Nonrecurring							(25.00%)
Measurement Input, Discount Rate [Member] | Long Lived Assets Held And Used [Member] | Valuation, Income Approach [Member] | Weighted Average [Member] | OPGC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Carrying Amount Disclosure Nonrecurring		$ 0.12					$ 0.12
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							5.20%
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							5.70%
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							1.80%

[1] Per the Company’s policy, pre-tax loss is limited to the impairment of long-lived assets. Any additional losses are recognized on completion of the sale. See Note 18—Held-for-Sale and Dispositions for further information.

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

[3] Includes $114 million of cumulative translation adjustment (debit balance) in the carrying value. Pre-tax loss is limited to the carrying value of the equity method investment excluding CTA.

[4] Excludes $115 million of cumulative translation adjustment (debit balance) in the carrying value.