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Summary of Significant Accounting Policies - Supplemental Cash Flow Information and Non-Cash Activity Related to Operating Leases (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Leases [Abstract]    
Cash paid for amounts included in the measurement of operating lease liability $ 15 $ 46
Right-of-use assets obtained in exchange for operating lease obligations $ 2 $ 381