Asset Impairment Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 0	$ 74		$ 116	$ 166
Kilroot and Ballylumford [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	0	0		115	0
Assets Carrying Amount Disclosure Nonrecurring	114			114
Shady Point [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	0	73	$ 83	0	156
Assets Carrying Amount Disclosure Nonrecurring		55	$ 127		55
Other Subsidiaries [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 0	$ 1		$ 1	$ 10