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Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restriced Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]        
Cash and cash equivalents $ 1,799 $ 831 $ 1,273  
Cash, cash equivalents, restricted cash and restricted cash equivalents 1,818 856 1,328 $ 1,275
Other current assets [Member]        
Restricted Cash and Cash Equivalents Items [Line Items]        
Restricted cash and cash equivalents 7 13 42  
Other noncurrent assets [Member]        
Restricted Cash and Cash Equivalents Items [Line Items]        
Restricted cash and cash equivalents $ 12 $ 12 $ 13