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Commitments - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Commitments [Line Items]      
Operating leases, rent expense $ 17.9 $ 20.0 $ 23.7
Floating Rate Reverse Mortgage Loans [Member]      
Other Commitments [Line Items]      
Additional borrowing capacity to borrowers 1,400.0    
Forward Mortgage Loan Interest Rate Lock Commitments [Member]      
Other Commitments [Line Items]      
Short-term commitments to lend 81.7    
Reverse Mortgage Loan Interest Rate Lock Commitments [Member]      
Other Commitments [Line Items]      
Short-term commitments to lend $ 14.6