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Employee Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended	24 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Employee Benefit Plans
Expense relating to the 401(k) plan	$ 785
Company obligations under Voluntary Deferred Compensation Plan for Executives		$ 3,000	$ 2,200
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%
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%
Participants vesting as percentage of discretionary matching contributions	100.00%
Expense relating to the 401(k) plan		$ 1,100

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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Balance Type:	na
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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Namespace Prefix:	osbc_
Data Type:	num:percentItemType
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ 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

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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:	num:percentItemType
Balance Type:	na
Period Type:	duration

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:	num:percentItemType
Balance Type:	na
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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:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: