Summary of Significant Accounting Policies (Details 5) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Numerator:
Net income									$ 50,182	$ 31,128	$ 8,875
Less amount allocable to participating securities									(454)	(320)
Net income available for basic shareholders	12,856	13,732	15,151	8,443	11,259	8,397	6,216	5,256	49,728	30,808	8,875
Adjustment for dilutive potential common shares									2	(2)
Net income available for diluted common shares									$ 49,730	$ 30,806	$ 8,875
Denominator:
Weighted average shares - Basic (in shares)									12,147,179	12,067,555	12,049,779
Adjustment for dilutive potential common shares (in shares)									68,852	82,311	109,750
Weighted average shares - Diluted (in shares)									12,216,031	12,149,866	12,159,529
Basic net income per share (in dollars per share)	$ 1.05	$ 1.12	$ 1.24	$ 0.69	$ 0.92	$ 0.70	$ 0.52	$ 0.44	$ 4.09	$ 2.55	$ 0.74
Diluted net income per share (in dollars per share)	$ 1.04	$ 1.11	$ 1.23	$ 0.68	$ 0.92	$ 0.69	$ 0.51	$ 0.43	$ 4.07	$ 2.54	$ 0.73
Number of stock option shares excluded as their effect would be anti-dilutive									98,630	130,195	216,224