Significant Accounting Policies (Cash and Restricted Cash) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 164	$ 66	$ 82
Restricted cash	22	23	20
Total cash, cash equivalents, and restricted cash shown in the Consolidated Statements of Cash Flows	$ 186	$ 89	$ 102	$ 113