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Junior Subordinated Debentures - Payments and other disclosures (Details) (USD $)	0 Months Ended	3 Months Ended			3 Months Ended	9 Months Ended
	Apr. 21, 2014	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Junior subordinated debentures	Sep. 30, 2014 Junior subordinated debentures item	Aug. 31, 2010 Junior subordinated debentures	Sep. 30, 2014 Junior subordinated debentures Old Second Capital Trust I	Sep. 30, 2014 Junior subordinated debentures Old Second Capital Trust II
Junior subordinated debentures
Junior subordinated debentures			$ 58,378,000	$ 58,378,000			$ 58,400,000	$ 32,600,000	$ 25,800,000
Period of interest payments that may be deferred (in quarters)						20
Payments to indentured trustees	$ 19,700,000	$ 19,700,000			$ 1,100,000

X

- Definition

The period up to which the interest payments may be deferred.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

The cash outflow for accrued and unpaid interest on outstanding Trust Preferred Securities.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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

X

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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