Mortgage Servicing Rights - Key Assumptions and the Sensitivity of the Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Transfers and Servicing [Abstract]
Aggregate portfolio principal balance	$ 439,561	$ 406,291
Weighted average rate of note	3.77%	3.77%
Conditional prepayment rate	8.20%	9.00%
Impact of 1 percent adverse change in prepayment speed, percent	21.33%	23.33%
Impact of 2 percent adverse change in prepayment speed, percent	23.12%	25.25%
Discount rate	9.73%	9.45%
Impact of 1 percent adverse change in discount rate, percent	8.73%	8.45%
Impact of 2 percent adverse change in discount rate, percent	7.73%	7.45%
Mortgage servicing rights	$ 8,039	$ 7,305
Impact of 1 percent adverse change, fair value of mortgage servicing rights	4,895	4,343
Impact of 2 percent adverse change, fair value of mortgage servicing rights	$ 4,648	$ 4,109
MSR as a percentage of serviced loans	1.83%	1.80%
Impact of 1 percent adverse change, percentage of mortgage servicing rights	1.11%	1.07%
Impact of 2 percent adverse change, percentage of mortgage servicing rights	1.06%	1.01%