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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator for diluted and basic earnings per share:
Net income	$ 34,987	$ 19,906	$ 77,344	$ 53,183
Denominator - share:
Basic weighted-average shares (in shares)	31,149	31,118	31,201	31,465
Effect of dilutive securities (in shares)	1,164	1,245	1,136	1,183
Diluted weighted-average shares (in shares)	32,313	32,363	32,337	32,648
Earnings per share:
Basic (in dollars per share)	$ 1.12	$ 0.64	$ 2.48	$ 1.69
Diluted (in dollars per share)	$ 1.08	$ 0.62	$ 2.39	$ 1.63