Significant Accounting Policies - Reconciliation of 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	$ 3,363	$ 3,315
Restricted cash and cash equivalents	772	779
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	$ 4,138	$ 4,097	$ 2,705	$ 2,917