Fair Value - Level 3 Unobservable Inputs, Mortgage Servicing Liabilities (Details) - Mortgage servicing liabilities - Level 3	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) 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 | $	$ 9,769,000	$ 14,120,000
Unpaid principal balance of underlying mortgage loans | $	$ 1,265,461,000	$ 1,620,609,000
Weighted-average servicing fee rate (as a percent)	25.00%	0.25%
Annual per-loan cost of servicing | $	$ 372	$ 404
Pricing spread
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	7.8	7.7
Annual total prepayment speed
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	31.6	32.9
Life
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	3.9	3.5