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]
Lease obligation period	5 years
Amortization of assets held under capital leases	$ 20.4	$ 35.5	$ 41.9