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FHLB Advances and Other Borrowings - Narrative (Details) - USD ($)
Mar. 31, 2020
Sep. 30, 2019
Debt Instrument [Line Items]    
Current borrowing capacity $ 1,460,000,000 $ 1,800,000,000
Loans pledged to the Federal Home Loan Bank 4,110,000,000 4,200,000,000
Revolving Credit Facility | FRB Discount Window Loan    
Debt Instrument [Line Items]    
Borrowing capacity 1,230,000,000 1,440,000,000
Loans pledged to the Federal Reserve Board Discount Window 1,480,000,000 1,720,000,000
Letter of Credit | Federal Home Loan Bank    
Debt Instrument [Line Items]    
Borrowing capacity 0.0 170,000,000.0
Additional Letters of Credit | Federal Home Loan Bank    
Debt Instrument [Line Items]    
Borrowing capacity $ 14,500,000 $ 14,900,000