Stock-Based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Options Outstanding
Beginning balance (in shares)	983,512
Granted (in shares)	82,580
Exercised (in shares)	(574,303)
Canceled (in shares)	(16,471)
Ending balance (in shares)	475,318	983,512
Exercisable at period end (in shares)	291,169
Weighted-Average Exercise Price per Share
Beginning balance (in dollars per share)	$ 40.62
Granted (in dollars per share)	77.54
Exercised (in dollars per share)	23.88
Canceled (in dollars per share)	64.46
Ending balance (in dollars per share)	56.50	$ 40.62
Exercisable (in dollars per share)	$ 49.75
Weighted-Average Remaining Contractual Life
Outstanding, weighted average remaining contractual life (in years)	5 years 1 month 299 days	4 years
Exercisable weighted average remaining contractual life (in years)	5 years
Aggregate Intrinsic Value
Beginning balance	$ 40,137
Granted	2,258
Exercised	46,520
Canceled	666
Ending balance	22,997	$ 40,137
Exercisable	$ 16,051