Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Commitments And Contingencies [Line Items]
Net rent expense under operating leases	$ 9.1	$ 8.9	$ 9.0
Loss Contingency Accrual	$ 0.0	$ 0.0
Maximum [Member]
Schedule Of Commitments And Contingencies [Line Items]
Term of lease	11 years