Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 4,357	$ 4,154	$ 3,196
Restricted cash and cash equivalents	78		70
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the consolidated statements of cash flows	$ 4,435	$ 4,223	$ 3,266	$ 2,508