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Borrowings - Additional information (Details)			3 Months Ended
Borrowings - Additional information (Details)	Mar. 02, 2020 USD ($)	Feb. 24, 2020 USD ($)	Mar. 31, 2020 USD ($) item	Dec. 31, 2016	Dec. 31, 2019 USD ($)	Apr. 20, 2018 USD ($)
Borrowings
Interest rate (as a percent)				5.75%
Bank owned FHLBC stock			$ 3,700,000
Senior notes			44,297,000		$ 44,270,000
Debt Instrument, Term				10 years
Interest rate term				5 years
Total borrowings			$ 154,290,000		205,870,000
GCFC/ABC Bank
Borrowings
Interest rate (as a percent)			2.83%
FHLBC advance amount			$ 6,000,000
Subordinated Debt.						$ 6,600,000
Debt assumed			23,400,000
Debt Instrument, Variable Rate Base LIBOR [Member]
Borrowings
Basis points added to reference rate (as a percent)				385.00%
Securities Sold under Agreements to Repurchase [Member]
Borrowings
Carrying amount of securities secured			51,200,000		48,700,000
Fair value of the pledged collateral			$ 71,200,000		70,700,000
Number of customers having secured balances exceeding specified percentage of stockholders equity \| item			0
Total borrowings			$ 51,236,000		48,693,000
Securities Sold under Agreements to Repurchase [Member] \| Minimum [Member]
Borrowings
Threshold percentage of stockholders' equity			10.00%
Federal Home Loan Bank Advances [Member]
Borrowings
Borrowings at FHLBC as percentage of total assets			35.00%
Borrowings at FHLBC as percentage of book value of certain mortgage loans			60.00%
FHLBC advance amount			$ 6,400,000		48,500,000
Bank owned FHLBC stock			3,700,000
Fair value of securities collateralized			51,000,000
Principal balance of loans collateralized			568,300,000
Combined collateral value			481,600,000
Amount available for additional borrowings			344,300,000
Total borrowings			6,375,000		48,500,000
FHLB advances at 1.76% to 2.23%
Borrowings
FHLBC advance amount			$ 6,400,000
FHLB advances at 1.76% to 2.23% \| Minimum [Member]
Borrowings
Interest rate (as a percent)			2.03%
FHLB advances at 1.76% to 2.23% \| Maximum [Member]
Borrowings
Interest rate (as a percent)			2.23%
Notes Payable to Banks [Member]
Borrowings
Total borrowings			$ 26,609,000		6,673,000
Credit Facility [Member]
Borrowings
Proceeds from term note		$ 20,000,000
Debt Instrument, Term		360 days
Junior Subordinated Debt
Borrowings
Total borrowings			25,773,000		57,734,000
Senior Notes.
Borrowings
Senior notes			44,297,000		44,270,000
Debt issuance costs			$ 703,000		$ 730,000
Amortization period for deferred financing costs			10 years
Term note
Borrowings
Proceeds from term note	$ 20,000,000	$ 20,000,000
Debt term		3 years
Term note \| Debt Instrument, Variable Rate Base LIBOR [Member]
Borrowings
Basis points added to reference rate (as a percent)		175.00%
Old Second Capital Trust I \| Junior Subordinated Debt
Borrowings
Cash distribution rate of trust preferred securities (as a percent)		7.80%