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Commitments and Contingencies - Narrative (Details) $ / shares in Units, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Oct. 11, 2018 USD ($) $ / shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Oct. 05, 2018 $ / shares shares
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Operating leases, rent expense |  | $ 3.2 | $ 2.5 | $ 1.9 |
Construction Contracts |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Commitments under construction contracts |  | $ 2.7 |  |  |
Related Entity |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Ownership percentage in partnership |  | 50.00% |  |  |
Idaho Independent Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Number of banking offices | 11 |  |  |  |
Estimate of Payments to Acquire Businesses, Gross | $ 181.3 |  |  |  |
Community 1st Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Number of banking offices | 3 |  |  |  |
Estimate of Payments to Acquire Businesses, Gross | $ 21.5 |  |  |  |
Inland Northwest Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Estimate value per share of Acquisition | $ / shares | $ 22.73 |  |  |  |
Community 1st Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Estimate value per share of Acquisition | $ / shares | $ 17.20 |  |  |  |
Class A Common Stock | Community 1st Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Estimate value per share of Acquisition | $ / shares |  |  |  |  | $ 45.45
Business Acquisition, Conversion Share Rate, Shares | shares |  |  |  |  | 0.3784
Class A Common Stock | Idaho Independent Bank [Member] |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Estimate value per share of Acquisition | $ / shares |  |  |  |  | $ 45.45
Business Acquisition, Conversion Share Rate, Shares | shares |  |  |  |  | 0.5000
Mortgage loans originated for sale |  |  |  |  |
Unrecorded Unconditional Purchase Obligation |  |  |  |  |
Guarantor Obligations, Current Carrying Value |  | $ 1.5 | $ 1.9 |  |

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