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																12 Months Ended
	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Numerator for basic and diluted earnings per share
Net income	$ 36,603		$ 20,548		$ 20,751		$ 16,977		$ 28,557		$ 19,622		$ 18,495		$ 23,421		$ 94,879	$ 90,095	$ 92,004
Denominator — share:
Basic weighted-average shares	32,123		34,210		34,500		34,699		35,132		35,499		35,664		35,043		33,870	35,332	34,909
Effect of dilutive securities																	994	960	1,154
Diluted weighted-average shares	33,217		35,162		35,311		35,820		36,151		36,385		36,492		36,151		34,864	36,292	36,063
Earnings per share:
Basic (in dollars per share)	$ 1.14	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 0.49	[1]	$ 0.81	[1]	$ 0.55	[1]	$ 0.52	[1]	$ 0.67	[1]	$ 2.80	$ 2.55	$ 2.64
Diluted (in dollars per share)	$ 1.10	[1]	$ 0.58	[1]	$ 0.59	[1]	$ 0.47	[1]	$ 0.79	[1]	$ 0.54	[1]	$ 0.51	[1]	$ 0.65	[1]	$ 2.72	$ 2.48	$ 2.55

[1] Earnings per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the totals for the respective years.