Stock Compensation - Summary of Option Activity in 2015 Plan, 2007 Plan and 1999 Plan (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Outstanding at beginning of year | shares	260,533
Shares, Granted | shares	50,000
Shares, Exercised | shares	(70,101)
Shares, Outstanding at end of period | shares	240,432
Shares, Shares subject to options exercisable at end of period | shares	190,432
Weighted average exercise price, Outstanding at beginning of year | $ / shares	$ 2.55
Weighted average exercise price, Granted | $ / shares	9.66
Weighted average exercise price, Exercised | $ / shares	3.31
Weighted average exercise price, Outstanding at end of period | $ / shares	3.81
Weighted average exercise price, Shares subject to options exercisable at end of period | $ / shares	$ 2.28
Aggregate intrinsic value, Outstanding at end of period | $	$ 1,408
Aggregate intrinsic value, Shares subject to options exercisable at end of period | $	$ 1,408