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Junior Subordinated Debentures - Issuance (Details)			1 Months Ended			6 Months Ended	7 Months Ended	12 Months Ended
	Jan. 28, 2020 USD ($)	Dec. 31, 2019 USD ($)	Apr. 30, 2007 USD ($)	Jul. 31, 2003 USD ($)	Jun. 30, 2003 USD ($)	Jun. 15, 2017	Dec. 31, 2017	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Junior subordinated debentures
Junior subordinated debentures		$ 57,734,000						$ 57,734,000	$ 57,686,000
Junior Subordinated Debentures issuance cost		644,000						644,000	692,000
Subordinated debentures		$ 57,734,000						$ 57,734,000	$ 57,686,000
Old Second Capital Trust I
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities				$ 4,100,000	$ 27,500,000
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 [Member]
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities			$ 25,000,000
Maturity Period								30 years
Cash distribution fixed rate of trust preferred securities (as a percent)		4.40%				6.77%			4.37%
Basis points added to cash distribution floating rate base (as a percent)							1.50%
Cash distribution, floating rate base									three-month LIBOR
Junior Subordinated Debt
Junior subordinated debentures
Junior subordinated debentures	$ 32,604,000
Interest Payment Deferment Period | item								20
Subordinated debentures	$ 32,604,000
Junior Subordinated Debt | Old Second Capital Trust I
Junior subordinated debentures
Cash distribution rate of trust preferred securities (as a percent)	7.80%
Junior subordinated debentures		$ 32,600,000						$ 32,600,000
Subordinated debentures		32,600,000						32,600,000
Redemption Price	$ 10.00
Redemption Price percentage of liquidation amount	100.00%
Junior Subordinated Debt | Old Second Capital Trust II [Member]
Junior subordinated debentures
Junior subordinated debentures		25,800,000						25,800,000
Subordinated debentures		$ 25,800,000						$ 25,800,000
7.8% subordinated debentures
Junior subordinated debentures
Interest rate (as a percent)	7.80%

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

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

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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.

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

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Represents the maturity period of trust preferred securities.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Percentage of principal amount of debt redeemed.

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

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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.

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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.

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