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Junior Subordinated Debentures - Issuance (Details)				1 Months Ended	6 Months Ended	7 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($)	Mar. 02, 2020 USD ($) $ / shares	Feb. 24, 2020 USD ($)	Apr. 30, 2007 USD ($)	Jun. 15, 2017	Dec. 31, 2017	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Jun. 16, 2017
Junior subordinated debentures
Cash distribution rate of trust preferred securities (as a percent)			7.80%
Junior subordinated debentures	$ 25,773,000						$ 25,773,000	$ 25,773,000
Junior Subordinated Debentures issuance cost	1,000						$ 1,000	1,000
Cash disbursed for redemption		$ 33,000,000.0
Redemption price per share | $ / shares		$ 10.13
Proceeds from term note	20,000,000.0		$ 20,000,000.0
Unamortized debt issuance costs		$ 635,000						$ 643,000	$ 48,000
Old Second Capital Trust II [Member]
Junior subordinated debentures
Proceeds from sale of cumulative trust preferred securities				$ 25,000,000.0
Maturity Period							30 years
Cash distribution fixed rate of trust preferred securities (as a percent)					6.77%
Basis points added to cash distribution floating rate base (as a percent)						1.50%
Cash distribution, floating rate base							three-month LIBOR
Term Debt
Junior subordinated debentures
Proceeds from term note	$ 13,000,000.0		$ 20,000,000.0
Junior subordinated debentures
Junior subordinated debentures
Interest Payment Deferment Period | item								20
Junior subordinated debentures | 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
Amortization period		30 years
Junior subordinated debentures | Old Second Capital Trust II [Member]
Junior subordinated debentures
Interest rate (as a percent)	4.40%						4.40%	4.40%		2.80%
Junior subordinated debentures							$ 25,800,000
7.8% subordinated debentures
Junior subordinated debentures
Interest rate (as a percent)		7.80%

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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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Amount of amortization expense attributable to debt issuance 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 outflow to reacquire preferred stocks issued 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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