Investments and Cash Investments and Cash - Cash and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Cash	$ 114	$ 144	$ 147	$ 118
Restricted cash	3	0	1	9
Cash and Restricted Cash	$ 117	$ 144	$ 148	$ 127