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Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Goodwill [Line Items] |  |
Property, Plant and Equipment, Net | $ 245.2 | $ 241.9
Goodwill [Roll Forward] |  |
Net carrying value at beginning of period | 444.7 | 212.8
Acquisitions and measurement period adjustments | 102.0 | 231.9
Net carrying value at end of period | 546.7 | $ 444.7
Inland Northwest Bank [Member] |  |
Goodwill [Line Items] |  |
Change in goodwill | $ 0.3 |

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