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Stock-Based Compensation - Summary of Restricted Stock Units Under Stock Plan (Detail) - Restricted Stock Units [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Restricted Stock Units, Outstanding Beginning Balance 720,840 345,555 5,160  
Number of Restricted Stock Units, Restricted stock granted 568,753 498,373 357,479  
Number of Restricted Stock Units, Restricted stock released (196,274) (83,121) (1,384)  
Number of Restricted Stock Units, Restricted stock cancelled (83,196) (39,967) (15,700)  
Number of Restricted Stock Units, Outstanding Ending Balance 1,010,123 720,840 345,555  
Number of Restricted Stock Units, Restricted stock expected to vest 930,205      
Weighted Average Grant Date Fair Value Outstanding, Beginning Balance $ 78.26 $ 70.39 $ 56.09  
Weighted Average Grant Date Fair Value, Granted 68.09 82.60 70.31  
Weighted Average Grant Date Fair Value, Released 77.43 69.90 56.65  
Weighted Average Grant Date Fair Value, Cancelled 81.73 81.57 65.15  
Weighted Average Grant Date Fair Value Outstanding, Ending Balance $ 72.41 $ 78.26 $ 70.39  
Aggregate Intrinsic Value Outstanding $ 39,284 $ 49,767 $ 25,108 $ 348
Aggregate Intrinsic Value, Released 968 $ 1,175 $ 115  
Aggregate Intrinsic Value, Expected to vest $ 36,176