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Fair Value Measurements - Quantitative and Qualitative Information (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014
Quantitative information about Level 3 fair value measurements |  |
Mortgage servicing rights, fair value | 5,254 us-gaap_ServicingAssetAtFairValueAmount | 5,462 us-gaap_ServicingAssetAtFairValueAmount
Recurring | Level 3 |  |
Quantitative information about Level 3 fair value measurements |  |
Mortgage servicing rights, fair value | 5,254 us-gaap_ServicingAssetAtFairValueAmount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 5,462 us-gaap_ServicingAssetAtFairValueAmount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Recurring | Mortgage servicing rights | Discounted Cash Flow | Level 3 |  |
Quantitative information about Level 3 fair value measurements |  |
Mortgage servicing rights, fair value | 5,254 us-gaap_ServicingAssetAtFairValueAmount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 5,462 us-gaap_ServicingAssetAtFairValueAmount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Mortgage servicing rights | Discounted Cash Flow | Level 3 | Minimum |  |
Quantitative information about Level 3 fair value measurements |  |
Discount rate (as a percent) | 9.70%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 9.70%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Prepayment speed (as a percent) | 4.80%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 5.00%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Mortgage servicing rights | Discounted Cash Flow | Level 3 | Maximum |  |
Quantitative information about Level 3 fair value measurements |  |
Discount rate (as a percent) | 15.50%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 108.20%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Prepayment speed (as a percent) | 35.10%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 78.40%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Mortgage servicing rights | Discounted Cash Flow | Level 3 | Weighted Average of Inputs |  |
Quantitative information about Level 3 fair value measurements |  |
Discount rate (as a percent) | 11.90%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 10.20%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Prepayment speed (as a percent) | 10.20%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember | 10.90%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Asset-backed securities | Discounted Cash Flow | Level 3 |  |
Quantitative information about Level 3 fair value measurements |  |
Asset-backed securities |  | 52,941 us-gaap_AssetBackedSecuritiesAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Asset-backed securities | Discounted Cash Flow | Level 3 | Minimum |  |
Quantitative information about Level 3 fair value measurements |  |
Credit risk premium (as a percent) |  | 0.90%osbc_FairValueInputsCreditRiskPremium / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Liquidity Discount (as a percent) |  | 3.50%osbc_FairValueInputsLiquidityDiscount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Asset-backed securities | Discounted Cash Flow | Level 3 | Maximum |  |
Quantitative information about Level 3 fair value measurements |  |
Credit risk premium (as a percent) |  | 0.90%osbc_FairValueInputsCreditRiskPremium / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Liquidity Discount (as a percent) |  | 3.70%osbc_FairValueInputsLiquidityDiscount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Recurring | Asset-backed securities | Discounted Cash Flow | Level 3 | Weighted Average of Inputs |  |
Quantitative information about Level 3 fair value measurements |  |
Credit risk premium (as a percent) |  | 0.90%osbc_FairValueInputsCreditRiskPremium / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember
Liquidity Discount (as a percent) |  | 3.60%osbc_FairValueInputsLiquidityDiscount / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = osbc_DiscountedCashFlowValuationTechniqueMember

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- Definition Represents the credit risk premium expressed as a percentage, used as an input to measure fair value. + References No definition available. + Details
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- Definition Represents the liquidity discount expressed as a percentage, used as an input to measure fair value. + References No definition available. + Details
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- Definition Investments in notes or bonds that are collateralized by a specific group of underlying assets owned by the issuer of the notes or bonds which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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Balance Type: | debit
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- Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122625-111746 + Details
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