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Securities Available for Sale - Narrative (Details)
12 Months Ended
Sep. 30, 2020
USD ($)
security
Sep. 30, 2019
USD ($)
security
Sep. 30, 2018
USD ($)
Investments, Debt and Equity Securities [Abstract]      
Proceeds from sale of AFS securities $ 151,899,000 $ 175,007,000 $ 24,971,000
Gross gains (pre-tax) 7,900,000 300,000 0.0
Gross losses (pre-tax) 0 500,000 0.0
Other-than-temporary impairment losses recognized in earnings 0 0 $ 0
Securities pledged as collateral $ 1,100,000,000 $ 863,900,000  
Percentage of investment portfolio in continuous loss position (as a percent) 6.00% 36.00%  
Number of securities in an unrealized loss position (number of securities) | security 18 169