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Regulatory & Capital Matters (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Apr. 30, 2014	Mar. 31, 2015	Dec. 31, 2014
Regulatory & Capital Matters
Tier 1 capital leverage ratio (as a percent)		9.82%us-gaap_TierOneLeverageCapitalToAverageAssets	9.93%us-gaap_TierOneLeverageCapitalToAverageAssets
Total capital ratio (as a percent)		17.30%us-gaap_CapitalToRiskWeightedAssets	17.68%us-gaap_CapitalToRiskWeightedAssets
Proceeds from issuance of common stock	$ 64.0us-gaap_SaleOfStockConsiderationReceivedOnTransaction
Old Second National Bank
Regulatory & Capital Matters
Tier 1 capital leverage ratio (as a percent)		12.15%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember	12.02%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember
Tier One leverage ratio, basis point increase (as a percent)		0.13%osbc_IncreaseInTierOneLeverageCapital / dei_LegalEntityAxis = us-gaap_SubsidiariesMember
Total capital ratio (as a percent)		18.18%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember	18.73%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember
Risk-based capital ratio, basis point increase (as a percent)		0.55%osbc_IncreaseInCapitalToRiskWeightedAssetsRatio / dei_LegalEntityAxis = us-gaap_SubsidiariesMember
Old Second National Bank | Minimum
Regulatory & Capital Matters
Tier 1 capital leverage ratio, board-designated threshold		8.00%osbc_BoardDesignatedTierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Total Capital Ratio, board-designated threshold (as a percent)		12.00%osbc_BoardDesignatedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = us-gaap_SubsidiariesMember / us-gaap_RangeAxis = us-gaap_MinimumMember

X

- Definition

Represents the board-designated threshold for Total Capital divided by risk weighted assets as defined in the regulations.

+ References

No definition available.

+ Details

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X

- Definition

Board-designated threshold for Tier 1 Leverage Capital divided by average assets as defined in the regulations.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
Data Type:	num:percentItemType
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X

- Definition

Represents the basis point increase in the risk-based capital ratio over prior period.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the basis point increase in the Tier One leverage capital over prior period.

+ References

No definition available.

+ Details

Name:	osbc_IncreaseInTierOneLeverageCapital
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Total Capital divided by risk weighted assets as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

+ Details

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- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

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

X

- Definition

Tier 1 Leverage Capital divided by average assets as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

+ Details

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Namespace Prefix:	us-gaap_
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X

- Details

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Namespace Prefix:
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