Equity Compensation Plans - Summary of Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares:
Outstanding at beginning of period (in shares)	427,000
Granted (in shares)	99,840	56,580	91,900
Exercised (in shares)	(90,897)	(76,333)	(166,175)
Forfeited/Cancelled (in shares)	(3,000)
Outstanding at end of period (in shares)	433,000	427,000
Options exercisable at end of period (in shares)	230,000
Unvested at end of period (in shares)	203,000
Weighted-Average Exercise Price:
Outstanding at beginning of period (in dollars per share)	$ 25.32
Granted (in dollars per share)	51.85
Exercised (in dollars per share)	14.87
Forfeited/Cancelled (in dollars per share)	64.20
Outstanding at end of period (in dollars per share)	33.37	$ 25.32
Options exercisable at end of period (in dollars per share)	22.11
Unvested at end of period (in dollars per share)	$ 46.05
Weighted- Average Remaining Contractual Term (years)
Outstanding at end of period	7 years 1 month
Options exercisable at end of period	5 years 5 months 19 days
Unvested at end of period	8 years 1 month
Aggregate Intrinsic Value
Outstanding at end of period	$ 8,116
Options exercisable at end of period	6,703
Unvested at end of period	$ 1,413