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Borrowings - Additional information (Details)		3 Months Ended			6 Months Ended
Borrowings - Additional information (Details)	Feb. 24, 2020 USD ($)	Jun. 30, 2022 USD ($) item	Jun. 30, 2021 USD ($)	Dec. 31, 2016	Jun. 30, 2022 USD ($) item	Dec. 31, 2021 USD ($)	Apr. 20, 2018 USD ($)
Borrowings
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		$ 5,500,000			$ 5,500,000	$ 7,100,000
Federal Home Loan Bank Chicago ("FHLBC") and Federal Reserve Bank Chicago ("FRBC") stock		20,413,000			20,413,000	13,257,000
Proceeds from term note	$ 30,000,000.0
Senior notes		44,533,000			44,533,000	44,480,000
Basis points added to reference rate (as a percent)	175.00%
Total borrowings		178,159,000			178,159,000	198,876,000
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
Carrying amount of securities secured		37,600,000			37,600,000	50,300,000
Fair value of the pledged collateral		$ 102,600,000			$ 102,600,000	113,000,000.0
Number of customers having secured balances exceeding specified percentage of stockholders equity \| item		0			0
Total borrowings		$ 37,599,000			$ 37,599,000	50,337,000
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		$ 0			$ 0	0
FHLBC stock		5,500,000			5,500,000
Principal balance of loans collateralized		820,300,000			820,300,000
Combined collateral value		559,300,000			559,300,000
Amount available for additional borrowings		479,300,000			479,300,000
Subordinated debentures
Borrowings
Total borrowings		59,254,000			59,254,000	59,212,000
Notes payable and other borrowings
Borrowings
Total borrowings		11,000,000			11,000,000	19,074,000
Junior subordinated debentures
Borrowings
Total borrowings		25,773,000			25,773,000	25,773,000
Senior notes
Borrowings
Interest rate (as a percent)				5.75%
Senior notes		44,500,000			44,500,000	44,500,000
Debt Instrument, Term				10 years
Interest rate term				5 years
Debt issuance costs		467,000			$ 467,000	520,000
Amortization period for deferred financing costs					10 years
Total borrowings		44,533,000			$ 44,533,000	$ 44,480,000
Debt instrument redemption price, percentage					100.00%
Senior notes \| 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				$ 11,000,000.0
Debt term	3 years
Subordinated Notes Due 2031
Borrowings
Interest rate (as a percent)			3.50%
Outstanding, net of deferred issuance costs		$ 59,300,000			$ 59,300,000
Face amount			$ 60,000,000.0