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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator for diluted and basic earnings per share:
Net income	$ 25,227	$ 34,987	$ 88,212	$ 77,344
Denominator - share:
Basic weighted-average shares (in shares)	30,914	31,149	30,973	31,201
Effect of dilutive securities (in shares)	1,310	1,164	1,367	1,136
Diluted weighted-average shares (in shares)	32,224	32,313	32,340	32,337
Earnings per share:
Basic (in dollars per share)	$ 0.82	$ 1.12	$ 2.85	$ 2.48
Diluted (in dollars per share)	$ 0.78	$ 1.08	$ 2.73	$ 2.39