Net Income Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income	$ 6,436	$ 15,151	$ 12,271	$ 23,594
Less amount allocable to participating securities	(51)	(136)	(97)	(212)
Net income available for basic shareholders	6,385	15,015	12,174	23,382
Adjustment for dilutive potential common shares		1		1
Net income available for diluted common shares	$ 6,385	$ 15,016	$ 12,174	$ 23,383
Denominator:
Weighted average shares - Basic	12,233,308	12,140,635	12,211,769	12,117,994
Adjustment for dilutive potential common shares	41,083	74,762	43,788	79,582
Weighted average shares - Diluted	12,274,391	12,215,397	12,255,557	12,197,576
Basic net income per share (in dollars per share)	$ 0.52	$ 1.24	$ 1.00	$ 1.93
Diluted net income per share (in dollars per share)	$ 0.52	$ 1.23	$ 0.99	$ 1.92
Number of stock option shares excluded as their effect would be anti-dilutive	184,706	80,380	184,706	80,380