Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 24,770	$ 18,457
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	18,700	15,900	$ 14,400
Acquisitions of mortgage servicing rights	3,500	0	300
Originations of mortgage servicing rights	5,600	5,800	3,600
Amortization expense	(3,000)	(3,000)	(2,400)
Servicing asset at amortized cost, before valuation reserve, ending	24,800	18,700	15,900
Less valuation reserve	0	(200)	(200)
Balance at end of year, net of valuation reserve	24,800	18,500	15,700
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,636,700	$ 3,127,500	$ 2,906,200
Mortgage servicing rights as a percentage of serviced loans	0.0068	0.0059	0.0054