Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 2,814	$ 3,186	$ 3,934
Restricted cash and cash equivalents	94		91
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the statements of cash flows	$ 2,908	$ 3,290	$ 4,025	$ 2,912