Capital Stock and Dividend Restrictions (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Apr. 08, 2019	Aug. 16, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 25, 2017
Class of Stock
Common stock, shares authorized			200,000,000
Stock compensation expense			$ 8.0	$ 5.6	$ 3.8
Aggregate purchase price of shares repurchased and retired			$ 2.5	$ 1.0	$ 1.3
Class A Common Stock
Class of Stock
Common stock, shares authorized			100,000,000
Common Stock, Voting Rights, Votes Per Share			$ 1
Common stock, shares, outstanding			43,129,085	38,169,575
Common shares purchased and retired			0
Class B Common Stock
Class of Stock
Common stock, shares authorized			100,000,000
Common Stock, Voting Rights, Votes Per Share			$ 5
Common stock, shares, outstanding			22,117,254	22,453,672
Community 1st Bank [Member]
Class of Stock
Total consideration paid	$ 18.8
Community 1st Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)	463,134
Idaho Independent Bank [Member]
Class of Stock
Total consideration paid	$ 157.3
Class A common stock	$ 157.3
Idaho Independent Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)	3,871,422
Inland Northwest Bank [Member]
Class of Stock
Total consideration paid		$ 176.3
Class A common stock		$ 173.3
Inland Northwest Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)		3,840,000
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period			22,417
Stock compensation expense			$ 0.8
Board of Directors
Class of Stock
Aggregate value of shares issued to directors				$ 0.5
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period				11,389
Secondary Public Offering | Class A Common Stock
Class of Stock
Common stock, value authorized						$ 250.0