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v2.4.1.9

Cash and Due from Banks (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Federal Reserve Bank regulatory reserve | Average |  |
Other disclosures |  |
Balances | $ 16.0 us-gaap_CompensatingBalanceAmount / us-gaap_CashAndCashEquivalentsAxis = osbc_FederalReserveBankRegulatoryReserveMember / us-gaap_RangeAxis = osbc_AverageMember | $ 8.5 us-gaap_CompensatingBalanceAmount / us-gaap_CashAndCashEquivalentsAxis = osbc_FederalReserveBankRegulatoryReserveMember / us-gaap_RangeAxis = osbc_AverageMember
Credit card processing services |  |
Other disclosures |  |
Balances | $ 4.4 us-gaap_CompensatingBalanceAmount / us-gaap_CashAndCashEquivalentsAxis = osbc_CreditCardProcessingServicesMember |

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- Definition The amount of a specific compensating balance arrangement that is maintained under an agreement for a bank loan or future credit availability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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