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Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018
Goodwill [Line Items] |  |  |
Goodwill, Acquired During Period |  | $ 74.9 | $ 102.0
Change in goodwill | $ 0.1 |  |
Property, Plant and Equipment, Net | 306.0 | 306.0 | 245.2
Goodwill [Roll Forward] |  |  |
Net carrying value at beginning of period |  | 546.7 | 444.7
Net carrying value at end of period | $ 621.6 | 621.6 | $ 546.7
Inland Northwest Bank [Member] |  |  |
Goodwill [Line Items] |  |  |
Change in goodwill |  | $ 0.3 |

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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
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- Definition Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(14)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(8)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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