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Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands |  |  | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
May 30, 2017 USD ($) $ / shares shares | Aug. 12, 2016 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Nov. 17, 2016 office | Dec. 31, 2014 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash received by Cascade shareholders (in dollars per share) | $ / shares | $ 1.91 |  |  |  |  |  |  |
Common stock issued (in shares) | shares | 11,300,000 |  |  |  |  |  |  |
Price per share of common stock issued (in dollars per share) | $ / shares | $ 34.3 |  |  |  |  |  |  |
Cash paid related to Cascade options and restricted stock | $ 9,300 |  |  |  |  |  |  |
Cash paid for Cascade unvested restricted stock | $ 2,200 |  |  |  |  |  |  |
Shares issued for cascade unvested restricted stock | shares | 168,000 |  |  |  |  |  |  |
Expense related to restricted awards excluded from consideration paid | $ 2,400 |  |  |  |  |  |  |
Restricted stock expense included In consideration paid | $ 5,500 |  |  |  |  |  |  |
Goodwill |  |  |  | $ 444,704 | $ 212,820 | $ 204,500 |  | $ 205,600
Number of operating segments | segment |  |  |  | 1 |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |  |  |  |
Acquisition related expenses |  |  |  | $ 27,156 | $ 2,821 | 795 |  |
Legal and professional fees directly related to merger |  |  |  | 6,500 |  |  |  |
Adjustment to pro forma net income for other costs |  |  |  | 21,800 |  |  |  |
Cascade Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of banking offices | office |  |  |  |  |  |  | 46 |
Core Deposits Intangibles |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Finite-lived intangible assets acquired | $ 48,000 | $ 2,500 |  |  |  | $ 700 |  |
Cascade Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Acquisition related expenses |  |  |  | 27,200 |  |  |  |
Legal and professional fees |  |  |  | $ 9,600 |  |  |  |
Revenue reported by Cascade |  |  | $ 12,900 |  |  |  |  |
Net income reported by Cascade |  |  | $ 3,000 |  |  |  |  |
Cascade Bank | Class A Common Stock |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Common stock received by Cascade shareholders (in shares) | shares | 0.14864 |  |  |  |  |  |  |
Common stock issued (in shares) | shares | 11,252,750 |  |  |  |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash paid as consideration | $ 155,000 |  |  |  |  |  |  |
Total consideration paid | 541,000 |  |  |  |  |  |  |
Goodwill | 231,900 |  |  |  |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company | Core Deposits Intangibles |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Finite-lived intangible assets acquired | $ 48,000 |  |  |  |  |  |  |
Flathead Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash paid as consideration |  | 34,100 |  |  |  |  |  |
Flathead Bank | Core Deposits Intangibles |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Finite-lived intangible assets acquired |  | $ 2,500 |  |  |  |  |  |
Estimated useful lives of related deposits |  | 9 years |  |  |  |  |  |

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