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Loans - Repossessed Assets (Details) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Real estate - residential |  |
Repossessed and foreclosed assets |  |
Real estate assets in process of foreclosure | $ 3,000,000 osbc_MortgageLoansInProcessOfForeclosureAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember | $ 3,500,000 osbc_MortgageLoansInProcessOfForeclosureAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialPortfolioSegmentMember
Other Assets |  |
Repossessed and foreclosed assets |  |
Repossessed assets | $ 0 us-gaap_OtherForeclosedAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember | $ 0 us-gaap_OtherForeclosedAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember

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- Definition Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process. + References No definition available. + Details
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Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount as of the balance sheet date of assets other than real property that were received in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |