Employee Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Expenses related to employer contributions	$ 3.0	$ 2.4	$ 1.9
Expenses related to deferred compensation plan	0.3	0.3	$ 0.3
Deferred compensation liability	$ 24.4	$ 21.1
Nonqualified Deferred Compensation Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Percent match by employer on the first 6% of employee compensation	50.00%
Maximum percent of compensation employer will match	3.00%