Cash and Cash Equivalents (Summary of Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash	$ 267	$ 261
Marketable securities	691	605
Other cash equivalents		50
Total	$ 958	$ 916	$ 1,382	$ 1,423