Investment Securities Realized Gains (Losses) on Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 156	$ 274	$ 49
Gross realized losses	19	$ 213	$ 48
State, county and municipal securities
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	140,458
State, county and municipal securities | Montana, Wyoming, and South Dakota
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	$ 78,337