Capital Stock and Dividend Restrictions (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Apr. 08, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 12, 2020	Jun. 11, 2019
Class of Stock
Common stock, shares authorized		200,000,000
Stock compensation expense		$ 7.5	$ 8.0	$ 5.6
Aggregate purchase price of shares repurchased and retired		$ 116.8	$ 2.5	$ 1.0
Stock Repurchase Program, Number of Shares Authorized to be Repurchased					5,500,000
Incremental Common Shares Attributable to Dilutive Effect of Accelerated Share Repurchase Agreements		3,000,000.0
Stock Repurchased and Retired During Period, Shares		3,578,743	43,560	24,271
Class A Common Stock
Class of Stock
Stock Repurchased and Retired During Period, Shares		3,538,142
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		40,335,113	43,129,085
Stock Repurchase Program, Number of Shares Authorized to be Repurchased						2,500,000
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		21,760,686	22,117,254
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
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period		19,491
Stock compensation expense		$ 0.6
Board of Directors
Class of Stock
Aggregate value of shares issued to directors			$ 0.8
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period			22,417