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Summary of Significant Accounting Policies - Dividend (Details) | 12 Months Ended
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Dec. 31, 2016
90-Days or Greater Past Due Loans |
Period from due date after which accrual of interest income is discontinued | 90 days

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- 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

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- References No definition available. + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentAgingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration