Lease Obligations - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Rent expense related to operating leases	$ 3,600,000	$ 2,500,000	$ 1,700,000
Operating Leases, Future Minimum Payments Due, Next Twelve Months	2,700,000
Operating Leases, Future Minimum Payments, Due in Two Years	1,700,000
Operating Leases, Future Minimum Payments, Due in Three Years	1,000,000
Operating Leases, Future Minimum Payments, Due in Four Years	815,000
Operating Leases, Future Minimum Payments, Due in Five Years	654,000
Future minimum payments under current lease agreements, thereafter	3,700,000
Future minimum payments under current lease agreements, total	10,600,000
Payments to Acquire Capital Leases	1,500,000
Capital Leases, Future Minimum Payments Due, Next Twelve Months	1,500,000
Capital Leases, Future Minimum Payments Due in Five Years	625,000
Capital Leases, Future Minimum Payments Due	$ 2,100,000