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Acquisitions - Business Combination (Narrative) (Details) |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |
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Apr. 25, 2018 USD ($) $ / shares | May 30, 2017 USD ($) $ / shares shares | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) segment | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Pro Forma Acquisition Expenses, Net of Tax |  |  |  | $ 10,700,000 |  | $ 11,200,000 |  |  |
Business acquisition cash conversion amount (in dollars per share) | $ / shares |  | $ 1.91 |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger | shares |  | 11,300,000 |  |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares |  | $ 34.30 |  |  |  |  |  |  |
Cash Paid Related To Cascade Options |  |  |  |  |  | 9,300,000 |  |  |
Cash Paid For Cascade Unvested Restricted Stock |  | $ 2,200,000 |  |  |  |  |  |  |
Shares Issued For Cascade Unvested Restricted Stock | shares |  | 168 |  |  |  |  |  |  |
Expense related to restricted awards excluded from consideration |  | $ 2,400,000 |  |  |  |  |  |  |
Restricted stock expense included in consideration |  |  |  |  |  | 5,500,000 |  |  |
Goodwill |  |  | $ 444,300,000 | 445,600,000 | $ 444,300,000 | $ 445,600,000 | $ 444,300,000 | $ 444,700,000 | $ 212,800,000
Number of operating segments | segment |  |  |  |  |  | 1 |  |  |
Acquisition related expenses |  |  |  | 0 | 10,100,000 | $ 2,300,000 | 10,800,000 |  |
Net income |  |  |  | $ 41,700,000 | $ 21,800,000 | $ 78,400,000 | $ 45,000,000 |  |
Inland Northwest Bank [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of banking offices acquired | 20 |  |  |  |  |  |  |  |
Estimate of Payments to Acquire Businesses, Gross | $ 159.7 |  |  |  |  |  |  |  |
Estimate value per share of Acquisition | $ / shares | $ 20.69 |  |  |  |  |  |  |  |
Bank of the Cascades |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of banking offices acquired |  | 46 |  |  |  |  |  |  |
Core Deposits |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimated useful lives of related deposits |  |  |  |  |  | 10 years |  |  |
Bank of the Cascades |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Revenues |  |  | 12,900,000 |  |  |  |  |  |
Net income |  |  | $ 3,000,000 |  |  |  |  |  |
Bank of the Cascades | Class A Common Stock |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business acquisition conversion share rate | shares |  | 0.14864 |  |  |  |  |  |  |
Bank of the Cascades | Core Deposits |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |  |  |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Core deposit intangible assets |  | $ 48,000,000 |  |  |  |  |  |  |
Cash paid by First Interstate in connection with merger |  | 155,000,000 |  |  |  |  |  |  |
Total consideration exchanged in connection with the merger |  | 541,000,000 |  |  |  |  |  |  |
Goodwill |  | $ 232,800,000 |  |  |  |  |  |  |

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