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Acquisition - Unaudited Pro Forma (Details) - West Suburban acquired loans - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Business Acquisition [Line Items] |  |
Net interest income | $ 166,495 | $ 165,283
Noninterest income | 59,036 | 48,937
Net income attributable to Old Second Bancorp, Inc. | $ 67,779 | $ 12,349

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