Deferred Compensation - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred compensation annual accrued interest rate	4.00%
Deferred compensation expense	$ 600	$ 700	$ 500
Deferred compensation	2,313	2,174
Key Employee
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred compensation paid	500
Research and Development Expenses
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred compensation expense	0	400	300
Selling, General, and Administrative Expenses
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred compensation expense	$ 600	$ 300	$ 200