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		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Numerator for diluted and basic earnings per share:
Net income	$ 23,116	$ 18,870	$ 42,357	$ 33,277
Denominator - share:
Basic weighted-average shares	31,268	31,335	31,226	31,639
Effect of dilutive securities	994	1,113	1,123	1,152
Diluted weighted-average shares	32,262	32,448	32,349	32,791
Earnings per share:
Basic (in dollars per share)	$ 0.74	$ 0.60	$ 1.36	$ 1.05
Diluted (in dollars per share)	$ 0.72	$ 0.58	$ 1.31	$ 1.01