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Acquisitions - Narrative (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 |  |
Acquisition related expenses |  | $ 0.0 | 20.3 | $ 12.4
Core Deposits Intangibles |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Finite-Lived Intangible Assets, Net | $ 3.0 | 51.2 | 62.1 |
Finite-lived intangible assets acquired |  | $ 0.0 | $ 16.6 |
Estimated useful lives of related deposits | 10 years |  |  |
Idaho Independent Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Core deposit intangible assets | $ 13.6 |  |  |
Number of banking offices | office | 11 |  |  |
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 [Member] | Core Deposits Intangibles |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |
Community 1st Bank [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities | $ 110.1 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets | $ 129.1 |  |  |
Number of banking offices | office | 3 |  |  |
Total consideration paid | $ 18.8 |  |  |
Goodwill | 2.3 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Assets | 16.5 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets | 78.8 |  |  |
Business Combination, Acquired Receivables, Estimated Uncollectible | 0.7 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Liabilities | $ 112.6 |  |  |
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 |  |  |

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- Definition For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=116869197&loc=d3e4845-128472 + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477 + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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