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Mortgage Servicing Rights (Key assumptions) (Details 4) - Mortgage Servicing Rights
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Key assumptions used in estimating the fair value of mortgage servicing rights    
Weighted-Average Constant Prepayment Rate (as a percent) 8.50% 8.13%
Weighted-Average Life (in years) 7 years 1 month 4 days 7 years 5 months 5 days
Weighted-Average Note Rate (as a percent) 4.04% 4.10%
Weighted-Average Discount Rate (as a percent) 8.87% 9.33%