Summary of Significant Accounting Policies - Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Net income	$ 5,633	$ 8,591	$ 5,877	$ 3,842	$ 5,189	$ 6,941	$ 4,979	$ 2,263	$ 23,943	$ 19,372	$ 22,988
Net income per share—Basic	$ 0.42	$ 0.65	$ 0.44	$ 0.29	$ 0.39	$ 0.53	$ 0.38	$ 0.17	$ 1.80	$ 1.48	$ 1.79
Net income per share—Diluted	$ 0.42	$ 0.64	$ 0.44	$ 0.29	$ 0.39	$ 0.52	$ 0.38	$ 0.17	$ 1.78	$ 1.46	$ 1.75
Average shares outstanding—Basic									13,272	13,096	12,820
Average shares outstanding—Diluted									13,439	13,267	13,120
Employee Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Incremental shares from the assumed exercise of outstanding stock options									108	122	239
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Incremental shares from the assumed lapse of restrictions on restricted stock awards									59	49	61