Property and Equipment - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Property and equipment, disposals recorded	$ 23	$ 50	$ 17
Pre-tax loss for disposed assets (less than)	(1)	(1)	(1)
Depreciation expense	$ 40	$ 38	$ 29