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Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Dec. 31, 2019
Accounting Policies [Abstract]        
Cash and cash equivalents $ 4,608 $ 4,154 $ 3,303  
Restricted cash and cash equivalents 68   63  
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the consolidated statements of cash flows $ 4,676 $ 4,223 $ 3,366 $ 2,508