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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Apr. 08, 2019 USD ($) office segment $ / shares shares | Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2019 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  | $ 621.6 |  | $ 621.6 | $ 621.6 | $ 621.6 | $ 546.7
Acquisition related expenses |  | $ 0.0 | $ 2.3 |  |  |  |
Core Deposits |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Amortization of Intangible Assets | $ 3.0 |  |  | $ 59.3 | $ 59.1 | $ 62.1 |
Estimated useful lives of related deposits | 10 years |  |  | 10 years |  |  |
Community 1st Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of banking offices acquired | office | 3 |  |  |  |  |  |
Total consideration exchanged | $ 18.8 |  |  |  |  |  |
Goodwill | 2.3 |  |  |  |  |  |
Provisional net assets acquired | 16.5 |  |  |  |  |  |
Provisional assets recorded | 129.1 |  |  |  |  |  |
Provisional loans recorded | 78.8 |  |  |  |  |  |
Acquired debt impaired | 0.7 |  |  |  |  |  |
Provisional liabilities recorded | 112.6 |  |  |  |  |  |
Provisional deposits recorded | $ 110.1 |  |  |  |  |  |
Community 1st Bank | Class A Common Stock |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares | 463,134 |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |  |
Business acquisition conversion share rate (in shares) | shares | 0.3784 |  |  |  |  |  |
Idaho Independent Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of banking offices acquired | office | 11 |  |  |  |  |  |
Total consideration exchanged | $ 157.3 |  |  |  |  |  |
Goodwill | $ 73.0 |  |  |  |  |  |
Number of operating segments | segment | 1 |  |  |  |  |  |
Core deposit intangible assets | $ 13.6 |  |  |  |  |  |
Idaho Independent Bank | Class A Common Stock |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares | 3,871,422 |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |  |
Business acquisition conversion share rate (in shares) | shares | 0.50 |  |  |  |  |  |
Idaho Independent Bank | Core Deposits |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |  |  |

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- Definition For loans and debt securities, 1) for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable, and 2) for which it is further probable that the investor will not be able to collect all cash flows expected at acquisition (plus any additional cash flows expected due to changes in estimate after acquisition), the allowance for credit losses recorded to reflect the impairment since acquisition. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5275-111524 + Details
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