Revenue (Narrative) (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Customer deposits	$ 900,000,000	$ 1,000,000,000.0
Long-term supply commitment	10 years
Credit memoranda issued	$ 37,000,000	0
Long-term liabilities	851,000,000	922,000,000
Other Noncurrent Liabilities [Member]
Customer deposits	851,000,000	922,000,000
Other Current Liabilities [Member]
Customer deposits	$ 42,000,000	$ 54,000,000