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Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Numerator:        
Net income $ 122,862 $ 120,440 $ 227,983 $ 185,500
Denominator:        
Shares used in net income per common share - basic 54,719,000 54,178,000 54,571,000 53,973,000
Effect of dilutive securities:        
Restricted stock units, stock appreciation rights and shares issued under employee stock purchase plan 555,000 823,000 709,000 1,006,000
Shares used in net income per common share - diluted 55,274,000 55,001,000 55,280,000 54,979,000
Net income per common share:        
Basic $ 2.25 $ 2.22 $ 4.18 $ 3.44
Diluted $ 2.22 $ 2.19 $ 4.12 $ 3.37