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Mortgage Loans - Mortgage Loans Held for Sale (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2017
Loans Receivable [Line Items]          
Mortgage loans held for sale $ 679.9   $ 679.9   $ 587.3
Aggregate outstanding principal amounts 659.2   659.2   570.8
Mortgage loans originated     5,500.0 $ 4,800.0  
Mortgage loans sold     5,400.0 4,800.0  
Net gain on sales of loans 68.0 $ 65.0 $ 193.7 $ 185.9  
Percentage of mortgage loans sold to significant purchasers     93.00%    
Percentage of mortgage loans sold to one significant purchaser     38.00%    
Uncommitted Loans [Member]          
Loans Receivable [Line Items]          
Mortgage loans held for sale 428.6   $ 428.6   330.7
Uncommitted Loans [Member] | Hedging Instruments related to IRLCs [Member]          
Loans Receivable [Line Items]          
Notional amounts of hedging instruments, Total $ 428.6   $ 428.6   $ 330.7