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Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Liabilities (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
item
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]    
Carrying value $ 7,844,000 $ 8,681,000
Mortgage servicing liabilities | Level 3    
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]    
Carrying value 7,844,000 8,681,000
Unpaid principal balance of underlying mortgage loans $ 1,000,403,000 $ 1,160,938,000
Servicing fee rate (as a percent) 0.0025 0.0025
Annual per-loan cost of servicing $ 364 $ 373
Mortgage servicing liabilities | Pricing spread | Level 3    
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]    
Input | item 7.4 7.3
Mortgage servicing liabilities | Annual total prepayment speed | Level 3    
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]    
Input | item 32.6 32.2
Mortgage servicing liabilities | Life | Level 3    
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]    
Input | item 3.6 3.8