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Fair Value Measurements - Changes in Level 3 (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest rate swap agreements net of swap credit valuation
Changes in Level 3 (liabilities)
Beginning balance	$ (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember
Included in earnings (or changes in net assets)	5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember
Ending balance	(1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember
Asset-backed securities
Changes in Level 3
Beginning balance	154,137us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember	52,941us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Total gains or losses
Included in earnings (or changes in net assets)	226us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember	(28)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Included in other comprehensive income	(315)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember	(541)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Purchases, issuances, sales, and settlements
Purchases	4,290us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Sales		(27,455)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Ending balance	158,338us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Transfers out of Level 3		(24,917)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
States and political subdivisions
Changes in Level 3
Beginning balance			118us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	125us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Purchases, issuances, sales, and settlements
Ending balance	125us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	118us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	118us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	125us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage servicing rights
Changes in Level 3
Beginning balance	5,807us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember	5,462us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember
Total gains or losses
Included in earnings (or changes in net assets)	111us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember	(454)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember
Purchases, issuances, sales, and settlements
Issuances	(304)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember	401us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember
Settlements		(155)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember
Ending balance	$ 5,614us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember	$ 5,254us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageServicingRightsMember

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