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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 |  |  |

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