Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 1,100	$ 400	$ 200
Gross realized losses	400	$ 100	$ 20
State, county and municipal securities
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	133,500
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	$ 108,200