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Financial Instruments - Reconciliation of recurring Level 3 fair value measurements (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 304 $ 0
Additions as a result of business combination 0 179
Purchases 75 186
Proceeds at maturity (90) (62)
Unrealized gains (losses) recognized in accumulated other comprehensive income (loss) (1) 1
Ending balance $ 288 $ 304