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Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 3,224 $ 2,826 $ 3,729  
Restricted cash and cash equivalents 74   95  
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the statements of cash flows $ 3,298 $ 2,926 $ 3,824 $ 3,290