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		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator for diluted and basic earnings per share:
Net income	$ 32,361	$ 25,227	$ 91,935	$ 88,212
Denominator - share:
Basic weighted-average shares (in shares)	29,708	30,914	29,924	30,973
Effect of dilutive securities (in shares)	1,453	1,310	1,417	1,367
Diluted weighted-average shares (in shares)	31,161	32,224	31,341	32,340
Earnings per share:
Basic (in dollars per share)	$ 1.09	$ 0.82	$ 3.07	$ 2.85
Diluted (in dollars per share)	$ 1.04	$ 0.78	$ 2.93	$ 2.73