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Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]        
Cash and cash equivalents $ 4,154 $ 2,452 $ 2,826  
Restricted cash and cash equivalents 69 56 100  
Total cash and cash equivalents, and restricted cash and cash equivalents presented in the consolidated statements of cash flows $ 4,223 $ 2,508 $ 2,926 $ 3,290