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Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 2,679 $ 2,826 $ 2,814  
Restricted cash and cash equivalents 55   94  
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the statements of cash flows $ 2,734 $ 2,926 $ 2,908 $ 3,290