Benefit Plans - Summary of Activity in Plans (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Outstanding at beginning of year | shares	371,218
Shares, Exercised | shares	(110,085)
Shares, Forfeited | shares	(600)
Shares, Outstanding at end of period | shares	260,533
Shares, Options exercisable at end of period | shares	260,533
Weighted average exercise price, Outstanding at beginning of year | $ / shares	$ 2.55
Weighted average exercise price, Exercised | $ / shares	2.53
Weighted average exercise price, Forfeited | $ / shares	2.10
Weighted average exercise price, Outstanding at end of period | $ / shares	2.55
Weighted average exercise price, Options exercisable at end of period | $ / shares	$ 2.55
Aggregate intrinsic value, Outstanding at end of period | $	$ 1,697
Aggregate intrinsic value, Options exercisable at end of period | $	$ 1,697