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Summarizes Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Stock Options Outstanding, beginning balance, Shares 2,103,470 1,530,845 1,674,534
Granted, Shares 808,500 862,000 524,534
Exercised, Shares (202,328) (112,875) (571,723)
Forfeited, Shares (57,500) (176,500) (96,500)
Stock Options Outstanding, ending balance, Shares 2,652,142 2,103,470 1,530,845
Stock Options Exercisable, Shares 984,374 675,152 439,561
Stock Options Outstanding, beginning balance, Weighted-Average Exercise Price $ 32.72 $ 27.46 $ 19.14
Granted, Weighted-Average Exercise Price 37.23 40.87 41.97
Exercised, Weighted-Average Exercise Price 13.62 13.24 16.21
Forfeited, Weighted-Average Exercise Price 42.54 39.32 28.47
Stock Options Outstanding, ending balance, Weighted-Average Exercise Price 35.34 32.72 27.46
Exercisable, Weighted-Average Exercise Price $ 29.84 $ 21.40 $ 12.18
Stock Options Outstanding, Weighted-Average Remaining Contractual Term 4 years 9 months 3 days 4 years 10 months 9 days 4 years 11 months 19 days
Exercisable, Weighted-Average Remaining Contractual Term 3 years 5 months 8 days 3 years 5 months 12 days 3 years 6 months 3 days
Stock Options Outstanding, Aggregate Intrinsic Value $ 6,964 $ 12,265 $ 30,573
Exercisable, Aggregate Intrinsic Value $ 6,534 $ 9,646 $ 15,480