Stock-Based Compensation - Schedule of Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total Stock Options
Options outstanding, beginning balance (in shares)	1,037,337
Options granted (in shares)	0	0
Options exercised (in shares)	(465,232)	(40,976)	0
Options forfeited and expired (in shares)	(6,680)
Options outstanding, ending balance (in shares)	565,425	1,037,337
Vested and expected to vest (in shares)	565,425
Exercisable (in shares)	459,653
Weighted- Average Exercise Price
Options outstanding, beginning balance, weighted average exercise price (in dollars per share)	$ 10.63
Options granted, weighted average exercise price (in dollars per share)	0.00
Options exercised, weighted average exercise price (in dollars per share)	6.98
Options forfeited and expired, weighted average exercise price (in dollars per share)	9.47
Options outstanding, ending balance, weighted average exercise price (in dollars per share)	13.65	$ 10.63
Vested and expected to vest, weighted average exercise price (in dollars per share)	13.65
Exercisable, weighted average exercise price (in dollars per share)	$ 14.94
Stock Option Activity, Additional Disclosures
Options outstanding, weighted average remaining contractual term (in years)	3 years 10 months 24 days
Vested and expected to vest, weighted average remaining contractual term (in years)	3 years 10 months 24 days
Exercisable, weighted average remaining contractual term (in years)	4 years 4 months 24 days
Options outstanding, aggregate intrinsic value	$ 4,150,751
Vested and expected to vest, aggregate intrinsic value	4,150,751
Exercisable, aggregate intrinsic value	$ 2,774,300
Time Based
Total Stock Options
Options outstanding, beginning balance (in shares)	818,331
Options granted (in shares)	0
Options exercised (in shares)	(338,133)
Options forfeited and expired (in shares)	(2,565)
Options outstanding, ending balance (in shares)	477,633	818,331
Vested and expected to vest (in shares)	477,633
Exercisable (in shares)	371,861
Performance Based
Total Stock Options
Options outstanding, beginning balance (in shares)	219,006
Options granted (in shares)	0
Options exercised (in shares)	(127,099)
Options forfeited and expired (in shares)	(4,115)
Options outstanding, ending balance (in shares)	87,792	219,006
Vested and expected to vest (in shares)	87,792
Exercisable (in shares)	87,792