Summary of Significant Accounting Policies - Reconciliation of Cash (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 1,755	$ 1,491
Restricted cash, current	117	115
Restricted cash, non-current	12	51
Total cash, cash equivalents, and restricted cash	$ 1,884	$ 1,657	$ 2,709	$ 4,658