Mortgage Servicing Rights - Narrative (Details) - Mortgage loans - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Mortgage servicing rights, fair value	$ 40,100,000	$ 35,700,000
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life	7 years 3 months 17 days	8 years
Impairment reversals	$ 90,000	$ 40,000	$ 100,000
Permanent impairment charged against carrying value	$ 0	$ 0	$ 0
Minimum
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Servicing assets at fair value, assumptions used to estimate fair value, discount rate	9.50%	9.60%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	0.50%	0.50%
Maximum
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Servicing assets at fair value, assumptions used to estimate fair value, discount rate	11.30%	11.40%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	1.80%	1.50%