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Junior Subordinated Debentures - Payments and other disclosures (Details) | 12 Months Ended
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Dec. 31, 2017 item
Junior subordinated debentures |
Junior subordinated debentures |
Period of interest payments that may be deferred (in quarters) | 20

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- 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

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- 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

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