Investments and Cash Investments and Cash - Cash and Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Cash	$ 82	$ 144	$ 72	$ 118
Restricted cash	0	0	0	9
Cash and Restricted Cash	$ 82	$ 144	$ 72	$ 127