Capital Stock and Dividend Restrictions (Details) $ in Millions		12 Months Ended
	May 30, 2017 USD ($) shares	Dec. 31, 2018 USD ($) votes shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Nov. 28, 2018 shares	Jun. 08, 2018 shares	Sep. 25, 2017 USD ($)
Class of Stock
Common stock, shares authorized		200,000,000
Aggregate purchase price of shares repurchased and retired | $		$ 1.0	$ 1.3	$ 26.9
Class A Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common stock, voting rights per share (vote per share) | votes		1
Common stock, shares, outstanding		38,169,575	33,560,202
Common shares purchased and retired		0	0
Class B Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common stock, voting rights per share (vote per share) | votes		5
Common stock, shares, outstanding		22,453,672	22,905,357
Community 1st Bank [Member] | Class A Common Stock
Class of Stock
Common stock, shares authorized					492,069
Idaho Independent Bank [Member] | Class A Common Stock
Class of Stock
Common stock, shares authorized					4,045,302
Inland Northwest Bank [Member] | Class A Common Stock
Class of Stock
Common stock, shares authorized						3,982,842
Cascade Bank | Class A Common Stock
Class of Stock
Common stock issued (in shares)	3,837,540
Class A common stock | $	$ 173.3
Board of Directors
Class of Stock
Aggregate value of shares issued to directors | $		$ 0.5	$ 0.5
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period		11,389	14,926
Secondary Public Offering | Class A Common Stock
Class of Stock
Common stock, value authorized | $							$ 250.0