Calculation of Weighted Average Common Shares and Common Equivalent Shares Outstanding (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Net income	$ 10,816	$ 16,869	$ 34,992	$ 38,882
Less: Income allocable to unvested restricted stock	(109)		(353)
Income available to common shareholders	10,707	16,869	34,639	38,882
Reallocation of participating earnings			1
Diluted income attributable to common shareholders	$ 10,707	$ 16,869	$ 34,640	$ 38,882
Weighted average number of common share equivalents:
Common shares used in basic earnings per share	12,950,416	12,882,396	12,948,788	12,854,858
Common share equivalents outstanding related to stock options	51,967	73,170	52,287	69,156
Total weighted average common shares and common share equivalents used in diluted earnings per share	13,002,383	12,955,566	13,001,075	12,924,014
Basic earnings per common share	$ 0.83	$ 1.31	$ 2.68	$ 3.02
Diluted earnings per common share	$ 0.82	$ 1.30	$ 2.66	$ 3.01