Annual report pursuant to Section 13 and 15(d)

Retirement Plan - Additional Information (Detail)

v3.8.0.1
Retirement Plan - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]      
Employer contribution, percentage 33.00% 33.00% 25.00%
Percentage of employee wage contributions matched by the employer 5.00% 5.00% 4.00%
Retirement Plan, Total expense $ 703,000 $ 505,000 $ 276,000