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Junior Subordinated Debentures - Payments and other disclosures (Details) $ in Millions |  | 12 Months Ended
--- | --- | ---
Apr. 21, 2014 USD ($) | Dec. 31, 2015 item
Junior subordinated debentures |  |
Payments to indentured trustees | $ | $ 19.7 |
Junior subordinated debentures |  |
Junior subordinated debentures |  |
Period of interest payments that may be deferred (in quarters) | item |  | 20

X
- Definition The period up to which the interest payments may be deferred. + References No definition available. + Details
Name: | osbc_InterestPaymentDefermentPeriod
Namespace Prefix: | osbc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash outflow for accrued and unpaid interest on outstanding Trust Preferred Securities. + References No definition available. + Details
Name: | osbc_PaymentsForAccruedAndUnpaidInterestOnOutstandingTrustPreferredSecurities
Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |