Mortgage Servicing Rights - Key Assumptions and the Sensitivity of the Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Transfers and Servicing [Abstract]
Aggregate portfolio principal balance	$ 516,008	$ 406,291
Weighted average rate of note	3.86%	3.77%
Conditional prepayment rate	8.28%	9.00%
Impact of 1 percent adverse change in prepayment rate, percent	21.26%	23.33%
Impact of 2 percent adverse change in prepayment rate, percent	23.04%	25.25%
Discount rate	9.89%	9.45%
Impact of 1 percent adverse change in discount rate, percent	8.89%	8.45%
Impact of 2 percent adverse change in discount rate, percent	7.89%	7.45%
Mortgage servicing rights	$ 9,695	$ 7,305
Impact of 1 percent adverse change, fair value of mortgage servicing rights	6,163	4,343
Impact of 2 percent adverse change, fair value of mortgage servicing rights	$ 5,896	$ 4,109
MSR as a percentage of serviced loans	1.88%	1.80%
Impact of 1 percent adverse change, percentage of mortgage servicing rights	1.19%	1.07%
Impact of 2 percent adverse change, percentage of mortgage servicing rights	1.14%	1.01%