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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, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Numerator:                      
Net income $ 71,636 $ 93,277 $ 122,862 $ 105,121 $ 77,638 $ 75,994 $ 120,440 $ 65,060 $ 392,896 $ 339,132 $ 104,809
Denominator:                      
Shares used in net income per common share - basic                 54,406,000 54,137,000 53,472,000
Effect of dilutive securities                 586,000 937,000 579,000
Shares used in net income per common share - diluted                 54,992,000 55,074,000 54,051,000
Net income per common share:                      
Basic $ 1.33 $ 1.71 $ 2.25 $ 1.93 $ 1.43 $ 1.40 $ 2.22 $ 1.21 $ 7.22 $ 6.26 $ 1.96
Diluted $ 1.32 $ 1.70 $ 2.22 $ 1.90 $ 1.41 $ 1.38 $ 2.19 $ 1.18 $ 7.14 $ 6.16 $ 1.94