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Mortgage Loans - Mortgage Loans Held for Sale Textual (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Loans Receivable [Line Items]      
Mortgage loans held for sale, principal amount $ 1,460.0 $ 1,040.0  
Mortgage loans held for sale 1,529.0 1,072.0  
Payments for Origination of Mortgage Loans Held-for-sale 12,200.0 8,700.0 $ 7,600.0
Proceeds from Sale of Mortgage Loans Held-for-sale 11,800.0 8,400.0 7,400.0
Net gain on sales of loans 437.2 319.4 $ 265.1
Mortgage servicing rights $ 17.1 0.0  
Other Customer [Member]      
Loans Receivable [Line Items]      
Percentage of Mortgage Loans Sold to Major Purchasers 28.00%    
Concentration Risk, Percentage 28.00%    
Loans Sold to FNMA or securities backed by GNMA [Member]      
Loans Receivable [Line Items]      
Percentage of Mortgage Loans Sold to Major Purchasers 66.00%    
Concentration Risk, Percentage 66.00%    
Uncommitted Loans [Member]      
Loans Receivable [Line Items]      
Mortgage loans held for sale $ 1,200.0 $ 663.8