Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Rent expense for noncancelable operating leases	$ 18.4	$ 15.1	$ 14.1
Minimum
Operating Leased Assets [Line Items]
Lease renewable period, maximum (in years)	1 year
Maximum
Operating Leased Assets [Line Items]
Lease renewable period, maximum (in years)	5 years