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Acquisition - Bancorp Financial - Schedule of Unaudited Pro Forma (Details) - Bancorp Financial - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Acquisition
Net interest income	$ 82,775	$ 75,221	$ 242,611	$ 222,009
Noninterest income	13,109	11,278	35,841	34,061
Net income attributable to Old Second Bancorp, Inc.	$ 29,586	$ 25,125	$ 80,941	$ 42,622

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
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-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-URI https://asc.fasb.org/1943274/2147479328/805-10-50-2

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