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Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024
Employee Benefit Plans
Expense relating to the 401(k) plan	$ 2.3	$ 2.2	$ 2.0
Company obligations under Voluntary Deferred Compensation Plan for Executives	$ 8.0	$ 5.9		$ 8.0
Old Second Bancorp Inc Employees 401(k) Savings Plan and Trust
Employee Benefit Plans
Company's discretionary matching contributions to the Plan (as a percent)	100.00%	100.00%	100.00%
Company's discretionary matching contributions to the Plan (as a percent)				50.00%
Percentage of compensation contributed to the Plan matched by employer (as a percent)				2.00%
Percentage of compensation contributed to the Plan matched by employer	3.00%	3.00%	3.00%
Participants vesting as percentage of discretionary matching contributions	100.00%

X

- Definition

Second percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Definition

Second percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements. Represents currently earned compensation under compensation arrangements that is not actually paid until a later date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480794/715-70-50-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480794/715-70-50-1

+ Details

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Balance Type:	na
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X

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
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X

- Definition

Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
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X

- Definition

Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
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X

- Details

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