Leases - Additional information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases, Rent Expense	$ 25.1	$ 20.8	$ 20.7
Operating Lease Period Minimum	3 years
Operating Lease Period Maximum	10 years
Minimum Miles Required per Year for the Fleet Lease	12,000
Minimum Period of the Fleet Lease	12 months
Fleet Lease's Expense	$ 1.2	$ 5.3	$ 7.2