Calculation of Weighted Average Common Shares and Common Equivalent Shares Outstanding (Detail) (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
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Net earnings available to common stockholders	$ 16,869	$ 11,431	$ 38,882	$ 20,116
Weighted average number of common share equivalents:
Common shares used in basic earnings per share	12,882,396	12,726,292	12,854,858	12,715,356
Common share equivalents outstanding related to stock options	73,170	134,488	69,156	124,204
Total weighted average common shares and common share equivalents used in diluted earnings per share	12,955,566	12,860,780	12,924,014	12,839,560
Basic earnings per common share	$ 1.31	$ 0.90	$ 3.02	$ 1.58
Diluted earnings per common share	$ 1.30	$ 0.89	$ 3.01	$ 1.57