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Cash and Due from Banks (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other disclosures
Balances	$ 11.0	$ 11.8
Federal Reserve Bank average regulatory reserve
Other disclosures
Balances	$ 10.6	$ 10.5

X

- Definition

The amount of average compensating balance arrangement that is maintained with the Reserve Bank, under an agreement for a bank loan or future credit availability.

+ References

No definition available.

+ Details

Name:	osbc_AverageReserveBalanceWithReserveBank
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- 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 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

Name:	us-gaap_CompensatingBalanceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

X

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=osbc_FederalReserveBankRegulatoryReserveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: