Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)	12 Months Ended			72 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining available shares for issuance under the plan	6,450,143
Stock Option Awards expiry range		5 years
Stock Option Awards granted				0
Options outstanding	120,000	970,000	1,085,000	120,000	1,120,000
Exercise price of options	$ 3.04			$ 3.04
Stock Option Awards expiration date	Mar. 17, 2018
Compensation expense to purchase shares under the ESPP	$ 299,000	$ 207,000	$ 144,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of authorize payroll deductions	15.00%			15.00%
Maximum Amount of outstanding stock to be subscribed	$ 25,000
Company reserved shares of common stock for issuance under the ESPP	600,000			600,000
Employees purchase shares	68,000	64,000	37,000
Compensation expense to purchase shares under the ESPP	$ 20,000	$ 19,000	$ 12,000
Shares remain available for issuance under the ESPP	118,456			118,456
Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Option Awards expiry range		10 years