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Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Apr. 08, 2019 USD ($) office $ / shares shares | Dec. 31, 2020 USD ($) segment | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Business Acquisition [Line Items] |  |  |  |
Goodwill |  | $ 621.6 | $ 621.6 | $ 546.7
Number of operating segments | segment |  | 1 |  |
Idaho Independent Bank, Fair Value Adjustments [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Cash and cash equivalents | $ 0.0 |  |  |
Investment securities | 0.5 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | (9.8) |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | 0.0 |  |  |
Allowance for loan losses | 6.3 |  |  |
Premises and equipment | 4.8 |  |  |
Other real estate owned (“OREO”) | 2.0 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 0.0 |  |  |
Core deposit intangible assets | 13.6 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | (2.6) |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 2.6 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | (0.7) |  |  |
Total assets acquired | 14.1 |  |  |
Deposits | 0.1 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 0.1 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 0.0 |  |  |
Total liabilities assumed | 2.8 |  |  |
Net assets acquired | $ 11.3 |  |  |
Idaho Independent Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of banking offices | office | 11 |  |  |
Cash and cash equivalents | $ 270.7 |  |  |
Investment securities | 63.2 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | 337.8 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | 0.5 |  |  |
Allowance for loan losses | 0.0 |  |  |
Premises and equipment | 21.3 |  |  |
Other real estate owned (“OREO”) | 2.4 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 15.2 |  |  |
Core deposit intangible assets | 13.6 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | 0.6 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 17.8 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | 7.9 |  |  |
Total assets acquired | 733.2 |  |  |
Deposits | 596.6 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 4.1 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 30.4 |  |  |
Total liabilities assumed | 648.9 |  |  |
Net assets acquired | 84.3 |  |  |
Class A common stock | 157.3 |  |  |
Total consideration paid | 157.3 |  |  |
Goodwill | $ 73.0 |  |  |
Idaho Independent Bank [Member] | Class A Common Stock |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Acquisition Conversion Share Rate | shares | 0.50 |  |  |
Common stock issued (in shares) | shares | 3,871,422 |  |  |
Business Acquisition, Share Price | $ / shares | $ 40.64 |  |  |
Idaho Independent Bank, As Recorded By Idaho Independent Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale | $ 0.5 |  |  |
Allowance for loan losses | (6.3) |  |  |
Premises and equipment | 16.5 |  |  |
Other real estate owned (“OREO”) | 0.4 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned | 15.2 |  |  |
Core deposit intangible assets | 0.0 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset | 3.2 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense | 15.2 |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets | 8.6 |  |  |
Total assets acquired | 719.1 |  |  |
Deposits | 596.5 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities | 4.0 |  |  |
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements | 30.4 |  |  |
Total liabilities assumed | 646.1 |  |  |
Net assets acquired | $ 73.0 |  |  |
Community 1st Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of banking offices | office | 3 |  |  |
Total consideration paid | $ 18.8 |  |  |
Goodwill | $ 2.3 |  |  |
Community 1st Bank [Member] | Class A Common Stock |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Acquisition Conversion Share Rate | shares | 0.3784 |  |  |
Common stock issued (in shares) | shares | 463,134 |  |  |
Business Acquisition, Share Price | $ / shares | $ 40.64 |  |  |
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Cash and cash equivalents | $ 270.7 |  |  |
Investment securities | 62.7 |  |  |
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment | $ 347.6 |  |  |
Core Deposits Intangibles |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |
Core Deposits Intangibles | Idaho Independent Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |

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