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Earnings per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Numerator for basic and diluted earnings per share                      
Net income $ 40,044 $ 25,227 $ 25,084 $ 37,901 $ 32,104 $ 34,987 $ 23,116 $ 19,241 $ 128,256 $ 109,448 $ 86,502
Denominator — share:                      
Basic weighted-average shares (in shares) 30,534 30,914 31,017 30,989 30,916 31,149 31,268 31,185 30,862 31,129 31,402
Effect of dilutive securities (in shares)                 1,383 1,179 1,207
Diluted weighted-average shares (in shares) 31,963 32,224 32,260 32,536 32,221 32,313 32,262 32,436 32,245 32,308 32,609
Earnings per share:                      
Basic (in dollars per share) $ 1.31 [1] $ 0.82 [1] $ 0.81 [1] $ 1.22 [1] $ 1.04 [1] $ 1.12 [1] $ 0.74 [1] $ 0.62 [1] $ 4.16 $ 3.52 $ 2.75
Diluted (in dollars per share) $ 1.25 [1] $ 0.78 [1] $ 0.78 [1] $ 1.16 [1] $ 1.00 [1] $ 1.08 [1] $ 0.72 [1] $ 0.59 [1] $ 3.98 $ 3.39 $ 2.65
[1] Earnings per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the totals for the respective years.