Summary of Significant Accounting Policies Reconciliation of Cash (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 1,491	$ 2,558
Restricted cash, current	115	116
Restricted cash, non-current	51	35
Total cash, cash equivalents, and restricted cash	$ 1,657	$ 2,709	$ 4,658	$ 1,414