Summary of 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	$ 5,464	$ 3,363
Restricted cash and cash equivalents	2,541	772
Restricted cash included within long-term investments and other assets	0	3
Total cash, cash equivalents and restricted cash and cash equivalents in the consolidated statement of cash flow	$ 8,005	$ 4,138	$ 6,366	$ 4,097