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Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Accounting Policies [Abstract]        
Cash and cash equivalents $ 3,303 $ 2,452 $ 2,253  
Restricted cash and cash equivalents 63   74  
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the consolidated statements of cash flows $ 3,366 $ 2,508 $ 2,327 $ 2,926