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Revenue (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Revenue [Abstract]    
Customer deposits $ 1,000 $ 1,000
Long-term supply commitment 10 years  
Credit memoranda issued $ 0 0
Long-term liabilities 907 922
Short-term liabilities $ 84 $ 54