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Securities - Asset-backed Securities (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Asset-backed securities |  |
Amortized Cost | $ 391,694 us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 376,286 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Fair Value | 385,486 us-gaap_AvailableForSaleSecurities | 372,191 us-gaap_AvailableForSaleSecurities
Asset-backed securities |  |
Asset-backed securities |  |
Percentage of original principal amount of the loans made under FFEL which is guaranteed by the U.S. Department of Education | 97.00%osbc_PercentageOfOriginalPrincipalAmountOfLoanUnderFfelWhichIsGuaranteedByUSDepartmentOfEducation / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Amortized Cost | 175,565 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 274,118 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Fair Value | 173,496 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 273,203 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Access Group |  |
Asset-backed securities |  |
Amortized Cost | 64,634 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentContractCounterpartyAxis = osbc_AccessGroupMember |
Fair Value | 64,155 us-gaap_AvailableForSaleSecurities / invest_InvestmentContractCounterpartyAxis = osbc_AccessGroupMember |
Northstar Education Finance |  |
Asset-backed securities |  |
Amortized Cost | 27,486 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentContractCounterpartyAxis = osbc_NorthstarEducationFinanceMember |
Fair Value | 27,526 us-gaap_AvailableForSaleSecurities / invest_InvestmentContractCounterpartyAxis = osbc_NorthstarEducationFinanceMember |
GCO Education Loan Funding Corp |  |
Asset-backed securities |  |
Amortized Cost | 38,469 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentContractCounterpartyAxis = osbc_GCOEducationLoanFundingCorpMember |
Fair Value | 37,315 us-gaap_AvailableForSaleSecurities / invest_InvestmentContractCounterpartyAxis = osbc_GCOEducationLoanFundingCorpMember |
Credit Concentration Risk | Stockholders' Equity | Asset-backed securities |  |
Asset-backed securities |  |
Number of issuers in portfolio | 3 osbc_NumberOfIssuersOfSecurityPortfolio / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Credit Concentration Risk | Stockholders' Equity | Access Group | Asset-backed securities |  |
Asset-backed securities |  |
Amortized Cost | 64,634 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_AccessGroupMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Fair Value | 64,155 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_AccessGroupMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Credit Concentration Risk | Stockholders' Equity | Northstar Education Finance | Asset-backed securities |  |
Asset-backed securities |  |
Amortized Cost | 27,486 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_NorthstarEducationFinanceMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Fair Value | 27,526 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_NorthstarEducationFinanceMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Credit Concentration Risk | Stockholders' Equity | GCO Education Loan Funding Corp | Asset-backed securities |  |
Asset-backed securities |  |
Amortized Cost | 38,469 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_GCOEducationLoanFundingCorpMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Fair Value | $ 37,315 us-gaap_AvailableForSaleSecurities / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_GCOEducationLoanFundingCorpMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Minimum | Credit Concentration Risk | Stockholders' Equity | Access Group | Asset-backed securities |  |
Asset-backed securities |  |
Credit risk as a percentage of benchmark | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_AccessGroupMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember |
Minimum | Credit Concentration Risk | Stockholders' Equity | Northstar Education Finance | Asset-backed securities |  |
Asset-backed securities |  |
Credit risk as a percentage of benchmark | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_NorthstarEducationFinanceMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember |
Minimum | Credit Concentration Risk | Stockholders' Equity | GCO Education Loan Funding Corp | Asset-backed securities |  |
Asset-backed securities |  |
Credit risk as a percentage of benchmark | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / invest_InvestmentContractCounterpartyAxis = osbc_GCOEducationLoanFundingCorpMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember |

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