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Borrowings - Additional information (Details)		3 Months Ended			9 Months Ended
Borrowings - Additional information (Details)	Feb. 24, 2020 USD ($)	Sep. 30, 2022 USD ($) item	Jun. 30, 2022 USD ($)	Dec. 31, 2016	Sep. 30, 2022 USD ($) item	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Apr. 20, 2018 USD ($)
Borrowings
Securities available-for-sale, at fair value		$ 1,609,759,000			$ 1,609,759,000	$ 1,693,632,000
Number of customers having secured balances exceeding specified percentage of stockholders equity \| item		0			0
Threshold percentage of stockholders' equity					10.00%
Borrowings at FHLBC as percentage of total assets		35.00%			35.00%
Borrowings at FHLBC as percentage of book value of certain mortgage loans		60.00%			60.00%
FHLBC stock		$ 4,500,000			$ 4,500,000	7,100,000
Federal Home Loan Bank Chicago ("FHLBC") and Federal Reserve Bank Chicago ("FRBC") stock		19,413,000			19,413,000	13,257,000
Proceeds from term note	$ 30,000,000.0
Senior notes		44,559,000			44,559,000	44,480,000
Basis points added to reference rate (as a percent)	175.00%
Total borrowings		200,104,000			200,104,000	198,876,000
Asset Pledged as Collateral with Right \| Securities sold under repurchase agreements
Borrowings
Investment securities pledged with financial institutions, dollars						50,300,000
Securities available-for-sale, at fair value		$ 72,400,000			72,400,000	113,000,000.0
GCFC/ABC Bank
Borrowings
Debt assumed								$ 23,400,000
Balance of borrowings paid off			$ 5,900,000
Three Months Secured Overnight Financing Rate
Borrowings
Basis points added to reference rate (as a percent)		273.00%
Securities sold under repurchase agreements
Borrowings
Total borrowings		$ 35,497,000			35,497,000	50,337,000
Securities sold under repurchase agreements \| Asset Pledged as Collateral with Right \| Securities sold under repurchase agreements
Borrowings
Investment securities pledged with financial institutions, dollars		$ 35,500,000			$ 35,500,000
Federal Home Loan Bank Advances
Borrowings
Borrowings at FHLBC as percentage of book value of certain mortgage loans		60.00%			60.00%
FHLBC advance amount		$ 25.0			$ 25.0	0
FHLBC stock		4,500,000			4,500,000
Principal balance of loans collateralized		861,100,000			861,100,000
Combined collateral value		576,700,000			576,700,000
Amount available for additional borrowings		536,700,000			536,700,000
Subordinated debentures
Borrowings
Total borrowings		59,275,000			59,275,000	59,212,000
Notes Payable to Banks [Member]
Borrowings
Total borrowings		10,000,000			10,000,000	19,074,000
Junior subordinated debentures
Borrowings
Total borrowings		25,773,000			25,773,000	25,773,000
Senior Notes [Member]
Borrowings
Interest rate (as a percent)				5.75%
Senior notes		44,600,000			44,600,000	44,500,000
Debt Instrument, Term				10 years
Interest rate term				5 years
Debt issuance costs		441,000			$ 441,000	520,000
Amortization period for deferred financing costs					10 years
Total borrowings		$ 44,559,000			$ 44,559,000	$ 44,480,000
Debt instrument redemption price, percentage					100.00%
Senior Notes [Member] \| Debt Instrument, Variable Rate Base LIBOR [Member]
Borrowings
Basis points added to reference rate (as a percent)				385.00%
Term Debt
Borrowings
Proceeds from term note	$ 20,000,000.0				$ 10,000,000.0
Debt term	3 years
Subordinated Notes Due 2031
Borrowings
Interest rate (as a percent)		3.50%			3.50%		3.50%
Outstanding, net of deferred issuance costs		$ 59,300,000			$ 59,300,000
Face amount							$ 60,000,000.0