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Junior Subordinated Debentures - Issuance (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended
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Jul. 31, 2003 Old Second Capital Trust I | Jun. 30, 2003 Old Second Capital Trust I | Sep. 30, 2014 Old Second Capital Trust I | Apr. 30, 2007 Old Second Capital Trust II | Sep. 30, 2014 Old Second Capital Trust II
Junior subordinated debentures |  |  |  |  |
Proceeds from sale of cumulative trust preferred securities | $ 4.1 | $ 27.5 |  | $ 25.0 |
Maturity Period |  |  | 30 years |  | 30 years
Cash distribution rate of trust preferred securities (as a percent) |  |  | 7.80% |  |
Cash distribution fixed rate of trust preferred securities (as a percent) |  |  |  |  | 6.77%
Cash distribution, floating rate base |  |  |  |  | three-month LIBOR
Basis points added to cash distribution floating rate base (as a percent) |  |  |  |  | 1.50%

X
- Definition The percentage points added to the reference rate to compute the variable rate on the trust preferred securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesBasisSpreadOnVariableRate
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the fixed annual rate of cash distributions on the securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesCashDistributionFixedRatePercentage
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the annual rate of cash distributions on the securities, which are payable quarterly. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesCashDistributionRatePercentage
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The reference rate for the variable rate of the trust preferred securities, such as LIBOR or the US treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesDescriptionOfVariableRateBasis
Namespace Prefix: | osbc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the maturity period of trust preferred securities. + References No definition available. + Details
Name: | osbc_TrustPreferredSecuritiesMaturityPeriod
Namespace Prefix: | osbc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash inflow from issuance of preferred stocks by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration