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Junior Subordinated Debentures - Issuance (Details) - USD ($) $ in Thousands	1 Months Ended			6 Months Ended	7 Months Ended	12 Months Ended
	Apr. 30, 2007	Jul. 31, 2003	Jun. 30, 2003	Jun. 15, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Junior subordinated debentures
Junior subordinated debentures					$ 57,639	$ 57,639	$ 57,591
Junior Subordinated Debentures issuance cost					$ 739	$ 739	$ 787
Old Second Capital Trust I
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities		$ 4,100	$ 27,500
Maturity Period						30 years
Cash distribution rate of trust preferred securities (as a percent)						7.80%
Amortization period						30 years
Old Second Capital Trust II
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities	$ 25,000
Maturity Period						30 years
Cash distribution fixed rate of trust preferred securities (as a percent)				6.77%	3.09%	4.30%
Basis points added to cash distribution floating rate base (as a percent)					1.50%
Cash distribution, floating rate base						three-month LIBOR
Junior subordinated debentures | Old Second Capital Trust I
Junior subordinated debentures
Junior subordinated debentures					$ 32,600	$ 32,600
Junior subordinated debentures | Old Second Capital Trust II
Junior subordinated debentures
Junior subordinated debentures					$ 25,800	$ 25,800

X

- Definition

The percentage points added to the reference rate to compute the variable rate on the trust preferred securities.

+ References

No definition available.

+ Details

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- Definition

Represents the fixed annual rate of cash distributions on the securities.

+ References

No definition available.

+ Details

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- Definition

Represents the annual rate of cash distributions on the securities, which are payable quarterly.

+ References

No definition available.

+ Details

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- Definition

The reference rate for the variable rate of the trust preferred securities, such as LIBOR or the US treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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- Definition

Represents the amortization period of the costs associated with the issuance of the trust preferred securities.

+ References

No definition available.

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- Definition

Represents the maturity period of trust preferred securities.

+ References

No definition available.

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- 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.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- 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 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: 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

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- Definition

The cash inflow from issuance of preferred stocks by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

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