Reconciliation of Basic and Diluted Earnings Per Share Attributable to Common Stock (Parenthetical) (Detail) (USD $)	3 Months Ended			6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012		Mar. 31, 2013	Mar. 31, 2012
Income Per Common Share [Line Items]
Basic earnings per share	$ 0.78	$ 0.71	[1]	$ 1.15	$ 1.11	[1]
Diluted earnings per share	$ 0.78	$ 0.70	[1]	$ 1.14	$ 1.11	[1]
Weighted average shares outstanding basic	7,861,204	7,841,362	[1]	7,855,284	7,836,787	[1]
Weighted average shares outstanding diluted	7,901,636	7,896,949	[1]	7,888,906	7,885,590	[1]
Scenario, Adjustment
Income Per Common Share [Line Items]
Basic earnings per share		$ 0.71			$ 1.13
Weighted average shares outstanding diluted		7,953,141			7,938,166

[1] Prior year earnings per share calculations have been adjusted for the two-class method to reflect restricted shares that were not reflected as participating in prior periods. Basic earnings per share as previously reported for the quarter and six months ended March 31, 2012 was $0.71 and $1.13, respectively. Diluted earnings per share as previously reported for the quarter and six months ended March 31, 2012 was $0.70 and $1.11, respectively. Basic weighted average shares outstanding as previously reported for the quarter and six months ended March 31, 2012 were 7,841,362 and 7,836,787, respectively. Diluted weighted average shares outstanding as previously reported for the quarter and six months ended March 31, 2012 were 7,953,141 and 7,938,166, respectively. The impact on all prior period financial statements is deemed immaterial.