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Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Apr. 08, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Mar. 31, 2019 | Mar. 31, 2018 | Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2019 | Jun. 11, 2019
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock Repurchased and Retired During Period, Value |  | $ 0.0 | $ 0.1 |  |  | $ 32.6 | $ 2.5 |  |  |  |
Stock repurchased and retired (in shares) |  |  |  | 607 | 39,360 |  | 896 | 22,596 |  |  |
Common Stock, Par or Stated Value Per Share |  | $ 29.56 |  |  |  | $ 29.56 |  |  |  |  |
Class A Common Stock |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares outstanding (in shares) |  | 42,579,590 |  |  |  | 42,579,590 |  |  |  | 43,129,085 |
Stock Repurchased and Retired During Period, Value |  |  |  |  |  | $ 31.2 |  |  |  |  |
Stock repurchased and retired (in shares) |  | 0 |  |  |  | 1,054,700 |  |  | 0 | 0 |
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased |  | 1,445,300 |  |  |  | 1,445,300 |  |  |  |  |
Stock Repurchase Program, Number of Shares Authorized to be Repurchased |  |  |  |  |  |  |  |  |  |  | 2,500,000
Class B Common Stock |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares outstanding (in shares) |  | 21,981,114 |  |  |  | 21,981,114 |  |  |  | 22,117,254 |
Idaho Independent Bank |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Value of common stock consideration issued | $ 157.3 |  |  |  |  |  |  |  |  |  |
Idaho Independent Bank | Class A Common Stock |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of shares issued as consideration (in shares) | 3,871,422 |  |  |  |  |  |  |  |  |  |
Community 1st Bank | Class A Common Stock |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of shares issued as consideration (in shares) | 463,134 |  |  |  |  |  |  |  |  |  |
Director [Member] | Class A Common Stock |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  | 19,200 |  |  |  | 19,200 |  |  |  |  |
Dividend, Share-based Payment Arrangement, Paid-in-Kind |  |  |  |  |  | $ 0.6 |  |  |  |  |

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