Investments and Cash - Cash and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Cash	$ 144	$ 118	$ 166	$ 75
Restricted Cash	0	9	0	19
Cash and Restricted Cash	$ 144	$ 127	$ 166	$ 94