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Loan Sales and Servicing - Valuation Assumptions (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Servicing Assets at Fair Value [Line Items]    
Fair value of MSRs $ 53,025 $ 49,597
Composition of residential loans serviced for others, percentage 100.00% 100.00%
Weighted average life (in years) 7 years 3 months 23 days 6 years 7 months 14 days
Prepayment speed: 7.90% 9.70%
Effect on fair value of a 10% increase $ (1,738) $ (1,582)
Effect on fair value of a 20% increase $ (3,371) $ (3,068)
Weighted average option adjusted spread: 8.20% 8.20%
Effect on fair value of a 10% increase $ (1,443) $ (1,568)
Effect on fair value of a 20% increase $ (2,798) $ (3,031)
Fixed rate mortgage loan    
Servicing Assets at Fair Value [Line Items]    
Composition of residential loans serviced for others, percentage 97.50% 97.40%
Adjustable rate mortgage loan    
Servicing Assets at Fair Value [Line Items]    
Composition of residential loans serviced for others, percentage 2.50% 2.60%