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Investment Securities - (Narrative) (Detail)
6 Months Ended
Mar. 31, 2021
USD ($)
Investment
Mar. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
Marketable Securities [Line Items]      
Sales $ 13,132,000 $ 5,045,000  
Gain on sale of securities available for sale 614,000 $ 180,000  
Fair value of available for sale securities transferred 3,800,000   $ 4,600,000
Fair value of available for sale securities short-term borrowings transferred 0   0
Carrying value of investment securities pledged against hedge $ 0   $ 0
U. S. Government Agencies [Member]      
Marketable Securities [Line Items]      
Number of portfolio investments | Investment 3    
Corporate Debt Securities [Member]      
Marketable Securities [Line Items]      
Number of portfolio investments | Investment 6    
Single Issuer Trust Preferred Security [Member]      
Marketable Securities [Line Items]      
Number of portfolio investments | Investment 1