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Significant Accounting Policies - Reconciliation of cash, cash equivalents and restricted cash (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 3,315 $ 2,443    
Restricted cash and cash equivalents 779 259    
Restricted cash included within long-term investments and other assets 3 3    
Total cash, cash equivalents and restricted cash and cash equivalents in the consolidated statement of cash flow $ 4,097 $ 2,705 $ 2,917 $ 1,818