Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	$ 22	$ 19	$ 16
Operating Leases, Future Minimum Payments Due, Next Twelve Months	9
Operating Leases, Future Minimum Payments, Due in Two Years	9
Operating Leases, Future Minimum Payments, Due in Three Years	8
Operating Leases, Future Minimum Payments, Due in Four Years	7
Operating Leases, Future Minimum Payments, Due in Five Years	7
Operating Leases, Future Minimum Payments, Due Thereafter	31
Operating Leases, Future Minimum Payments Due	$ 71