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Acquisition - Additional Information (Details) - USD ($) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
May 15, 2019 | Apr. 30, 2019 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2018
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Goodwill |  |  | $ 145,638,000 | $ 148,353,000 | $ 145,638,000 | $ 148,353,000 | $ 148,353,000 | $ 145,638,000 | $ 128,177,000
Oak Park River Forest Bankshares, Inc. |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business acquisition date of agreement |  |  |  | Apr. 30, 2019 |  |  |  |  |
Right to receive common stock upon conversion |  | 7.9321 |  |  |  |  |  |  |
Consideration paid in cash | $ 4,200,000 | $ 6,163,000 |  |  |  |  |  |  |
Common stock outstanding value per share |  | $ 33.375 |  |  |  |  |  |  |
Common stock issued price per share |  | $ 20.02 |  |  |  |  |  |  |
Issuance of common shares |  | 1,464,558 |  |  |  |  |  |  |
Value of common stock consideration |  | $ 29,300,000 |  |  |  |  |  |  |
Options to acquire common stock shares |  | 35,870 |  |  |  |  |  |  |
Option to acquire common stock value |  | $ 4,200,000 |  |  |  |  |  |  |
Total merger consideration |  | 35,483,000 |  |  |  |  |  |  |
Stock issuance costs |  | 585,000 |  |  |  |  |  |  |
Goodwill |  | $ 20,176,000 |  |  |  |  |  |  |
Oak Park River Forest Bankshares, Inc. | Non-interest Expense |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Acquisition advisory expenses |  |  |  |  | 2,300,000 | 2,300,000 |  |  |
Core system conversion expenses |  |  | 1,200,000 |  | $ 1,900,000 | $ 2,000,000.0 |  |  |
Merger related income from legal fee credit |  |  | $ 135,000 |  |  |  |  |  |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e1043-128460 + Details
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