Employee Benefit Plans - Employee 401(k) Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage employee contribution matched by the Company	50.00%
Maximum employee contribution matched by the Company	4.00%
401k vesting period	4 years
Employee contribution matched by the company	$ 2.3	$ 2.2	$ 2.3
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees contribution out of annual compensation	1.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees contribution out of annual compensation	75.00%