Summary of Significant Accounting Policies - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash and cash equivalents	$ 535	$ 407	$ 627
Short-term restricted cash and cash equivalents	769	679	657
Long-term restricted cash and cash equivalents	264	264	263
Total cash, cash equivalents and restricted cash and cash equivalents shown in our statements of cash flows	$ 1,568	$ 1,350	$ 1,547	$ 1,278