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Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 3,729 $ 3,186 $ 3,198  
Restricted cash and cash equivalents 95   95  
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the statements of cash flows $ 3,824 $ 3,290 $ 3,293 $ 2,912