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Summary of Significant Accounting Policies - Securities, Receivables, and Servicing Assets (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 item | Dec. 31, 2013
90-Days or Greater Past Due Loans |  |
Period from due date after which accrual of interest income is discontinued | 90 days |
Allowance for Loan Losses |  |
Number of management factors of methodology calculation subject to minor changes | 2 osbc_NumberOfManagementFactorsOfMethodologyCalculationsSubjectToMinorChange |
Transfers and Servicing of Financial Assets |  |
Mortgage loans serviced for others | $ 604.2 osbc_MortgageLoansServicingAssetAtFairValueAmount | $ 591.5 osbc_MortgageLoansServicingAssetAtFairValueAmount

X
- Definition Fair value as of the balance sheet date of mortgage loans representing net future revenues from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements. + References No definition available. + Details
Name: | osbc_MortgageLoansServicingAssetAtFairValueAmount
Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the number of management factors included in the methodology calculations that are subject to minor changes during the period. + References No definition available. + Details
Name: | osbc_NumberOfManagementFactorsOfMethodologyCalculationsSubjectToMinorChange
Namespace Prefix: | osbc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period from due date after which accrual of interest income is discontinued when full loan repayment is in doubt or when a loan becomes contractually past due. + References No definition available. + Details
Name: | osbc_PeriodFromDueDateAfterWhichAccrualOfInterestIncomeIsDiscontinued
Namespace Prefix: | osbc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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

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