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Borrowings - Additional information (Details)			12 Months Ended
	Dec. 31, 2020 USD ($) item	Feb. 24, 2020 USD ($)	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Borrowings
Threshold percentage of stockholders' equity			10.00%
Borrowings at FHLBC as percentage of total assets	35.00%		35.00%
FHLBC advance amount					$ 6,700,000
Bank owned FHLBC stock	$ 3,700,000		$ 3,700,000
Fair value of securities collateralized	54,700,000		54,700,000
Principal balance of loans collateralized	625,800,000		625,800,000
Maximum borrowing capacity				$ 459,500,000
Proceeds from term note	20,000,000.0	$ 20,000,000
Cash distribution rate of trust preferred securities (as a percent)		7.80%
Amount available for additional borrowings	336,900,000		336,900,000
Senior notes	44,375,000		44,375,000	44,270,000
Basis points added to reference rate (as a percent)		175.00%
Debt issuance costs				1,000	644,000
Total borrowings	160,521,000		160,521,000	$ 205,870,000
GCFC/ABC Bank
Borrowings
FHLBC advance amount	$ 6,400,000		$ 6,400,000
Debt Instrument, Term			5 years 3 months	6 years 3 months
Debt assumed					$ 23,400,000
GCFC/ABC Bank | Maximum [Member]
Borrowings
Interest rate (as a percent)	2.83%		2.83%	2.83%
Securities Sold under Agreements to Repurchase [Member]
Borrowings
Carrying amount of securities secured	$ 67,000,000.0		$ 67,000,000.0	$ 48,700,000
Fair value of the pledged collateral	$ 94,400,000		$ 94,400,000	70,700,000
Number of customers having secured balances exceeding specified percentage of stockholders equity | item	0		0
Total borrowings	$ 66,980,000		$ 66,980,000	48,693,000
Securities Sold under Agreements to Repurchase [Member] | Minimum [Member]
Borrowings
Maturity			1 day
Securities Sold under Agreements to Repurchase [Member] | Maximum [Member]
Borrowings
Maturity			90 days
Federal Home Loan Bank Advances [Member]
Borrowings
FHLBC advance amount	0		$ 0	48,500,000
Total borrowings				$ 48,500,000
FHLB advances at 1.44 %
Borrowings
Interest rate (as a percent)				1.78%
Notes Payable to Banks [Member]
Borrowings
Total borrowings	23,393,000		23,393,000	$ 6,673,000
Junior Subordinated Debt
Borrowings
Total borrowings	$ 25,773,000		$ 25,773,000	57,734,000
Senior Notes.
Borrowings
Interest rate (as a percent)	5.75%		5.75%
Senior notes	$ 44,400,000		$ 44,400,000	44,300,000
Debt Instrument, Term			10 years
Interest rate term			5 years
Debt issuance costs	625,000		$ 625,000	730,000
Amortization period for deferred financing costs			10 years
Total borrowings	44,375,000		$ 44,375,000	$ 44,270,000
Senior Notes. | Debt Instrument, Variable Rate Base LIBOR [Member]
Borrowings
Basis points added to reference rate (as a percent)				385.00%
Term note
Borrowings
Proceeds from term note	$ 17,000,000.0	$ 20,000,000.0

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Amount of long-term debt due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

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