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Net Income Per Share - Reconciliation of Basic and Diluted Net Income Per Share (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Numerator:                      
Net income $ 77,638 $ 75,994 $ 120,440 $ 65,060 $ 45,487 $ 32,549 $ 9,210 $ 17,563 $ 339,132 $ 104,809 $ 122,297
Denominator:                      
Shares used in net income per common share - basic                 54,137,000 53,472,000 53,282,000
Effect of dilutive securities                 937,000 579,000 278,000
Shares used in net income per common share - diluted                 55,074,000 54,051,000 53,560,000
Net income per common share:                      
Basic $ 1.43 $ 1.40 $ 2.22 $ 1.21 $ 0.85 $ 0.61 $ 0.17 $ 0.33 $ 6.26 $ 1.96 $ 2.30
Diluted $ 1.41 $ 1.38 $ 2.19 $ 1.18 $ 0.83 $ 0.60 $ 0.17 $ 0.33 $ 6.16 $ 1.94 $ 2.28