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
	Dec. 31, 2017	Dec. 31, 2016
Numerator for diluted and basic earnings per share:
Net income	$ 27,299	$ 37,901
Denominator - share:
Basic weighted-average shares (in shares)	30,078	30,989
Effect of dilutive securities (in shares)	1,483	1,547
Diluted weighted-average shares (in shares)	31,561	32,536
Earnings per share:
Basic (in dollars per share)	$ 0.91	$ 1.22
Diluted (in dollars per share)	$ 0.86	$ 1.16