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Mortgage Banking Derivatives (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Mortgage Banking Derivatives |  |  |
Amount of loans sold to investors | $ 120,900,000 osbc_AmountOfLoansHeldForSaleSold |  |
Proceeds from sale of loan to investors | 124,458,000 us-gaap_ProceedsFromSaleOfLoansHeldForSale | 191,019,000 us-gaap_ProceedsFromSaleOfLoansHeldForSale | 303,561,000 us-gaap_ProceedsFromSaleOfLoansHeldForSale
Gain on sale of loan | 3,594,000 us-gaap_GainLossOnSaleOfMortgageLoans | 5,627,000 us-gaap_GainLossOnSaleOfMortgageLoans | 10,688,000 us-gaap_GainLossOnSaleOfMortgageLoans
Federal National Mortgage Association |  |  |
Mortgage Banking Derivatives |  |  |
Amount of loans sold to investors | 79,500,000 osbc_AmountOfLoansHeldForSaleSold / us-gaap_MajorCustomersAxis = osbc_FederalNationalMortgageAssociationMember |  |
Federal National Mortgage Association | Customer concentration risk |  |  |
Mortgage Banking Derivatives |  |  |
Percentage of concentration risk | 65.60%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = osbc_FederalNationalMortgageAssociationMember |  |
Wells Fargo |  |  |
Mortgage Banking Derivatives |  |  |
Amount of loans sold to investors | 23,600,000 osbc_AmountOfLoansHeldForSaleSold / us-gaap_MajorCustomersAxis = osbc_WellsFargoMember |  |
Wells Fargo | Customer concentration risk |  |  |
Mortgage Banking Derivatives |  |  |
Percentage of concentration risk | 19.50%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = osbc_WellsFargoMember |  |
Collateralized mortgage obligation ("MBS") |  |  |
Mortgage Banking Derivatives |  |  |
Gain on sale of loan | 143,000 us-gaap_GainLossOnSaleOfMortgageLoans / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_MortgageBackedSecuritiesMember | 315,000 us-gaap_GainLossOnSaleOfMortgageLoans / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_MortgageBackedSecuritiesMember | 567,000 us-gaap_GainLossOnSaleOfMortgageLoans / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_MortgageBackedSecuritiesMember
Not designated | Forward MBS |  |  |
Mortgage Banking Derivatives |  |  |
Notional or Contractual Amount | 14,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 11,500,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Fair value | 615,000 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 178,000 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Change in fair value | (79,000)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 126,000 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Not designated | Rate Lock Commitments |  |  |
Mortgage Banking Derivatives |  |  |
Notional or Contractual Amount | 10,876,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 9,178,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Fair value | 509,000 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 504,000 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Change in fair value | $ 222,000 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | $ 189,000 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateLockCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Represents the amount of loans held for sale, sold during the period. + References No definition available. + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition The gains (losses) included in earnings that represent the difference between the sale price and the carrying value of loans made to finance real estate acquisitions. This element refers to the gain (loss) and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition The cash inflow resulting from the sale of loans classified as held-for-sale, including proceeds from loans sold through mortgage securitization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 21 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3461-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 230 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=34726456&loc=d3e60097-112785 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3337-108585 + Details
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