Annual report pursuant to Section 13 and 15(d)

Stock Option and Purchase Plans - Summary of Company's Stock Option Activity and Related Information (Detail)

v3.8.0.1
Stock Option and Purchase Plans - Summary of Company's Stock Option Activity and Related Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options      
Balance at beginning of the period (in shares) 1,338,273 1,444,954 1,686,178
Options granted (in shares) 103,140 104,900 105,742
Options exercised (in shares) (114,699) (188,768) (346,966)
Options forfeited (in shares) (16,624) (22,813) 0
Balance at end of the period (in shares) 1,310,090 1,338,273 1,444,954
Exercisable at end of the period (in shares) 1,088,970 1,091,561 1,167,040
Weighted Average Exercise Price      
Balance at beginning of the period (in usd per share) $ 14.85 $ 12.61 $ 9.43
Options granted (in usd per share) 38.41 34.29 35.80
Options exercised (in usd per share) 11.24 7.20 4.25
Options forfeited (in usd per share) 27.91 25.96 0.00
Balance at end of the period (in usd per share) 16.85 14.85 12.61
Exercisable at end of the period (in usd per share) $ 12.84 $ 11.03 $ 9.20
Aggregate Intrinsic Value      
Balance at beginning of the period (in usd) $ 35,630 $ 30,675 $ 43,778
Options granted (in usd) 315 (48) (42)
Options exercised (in usd) (3,467) (5,029) (10,808)
Options forfeited (in usd) (225) (180) 0
Balance at end of the period (in usd) 32,253 25,418 32,928
Exercisable at end of the period (in usd) $ 31,177 $ 24,898 $ 30,576