Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Numerator for diluted and basic earnings per share:
Net income	$ 18,870	$ 20,751	$ 33,277	$ 37,728
Denominator - share:
Basic weighted-average shares	31,335	34,500	31,639	34,705
Effect of dilutive securities	1,113	811	1,152	965
Diluted weighted-average shares	32,448	35,311	32,791	35,670
Earnings per share:
Basic (in dollars per share)	$ 0.60	$ 0.60	$ 1.05	$ 1.09
Diluted (in dollars per share)	$ 0.58	$ 0.59	$ 1.01	$ 1.06