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Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2025 USD ($) Security	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) Security	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) Security
Debt Securities, Available-for-Sale [Line Items]
Proceeds from sale of debt securities available-for-sale	$ 0	$ 0	$ 0	$ 0
Gross realized gains	0	0	0	0
Gross realized losses	0	$ 0	0	$ 0
Carrying value of pledged securities	$ 235,206,000		$ 235,206,000		$ 206,600,000
Number of securities in portfolio with unrealized losses | Security	427		427		508
Available for sale term	3 years		3 years
Equity securities	$ 2,600,000		$ 2,600,000		$ 2,400,000
FHLBC stock	16,100,000		16,100,000		18,500,000
FRBC stock	11,500,000		11,500,000		11,500,000
Other securities	27,901,000		27,901,000		30,352,000
United Bankers Bancorp [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	225,000		225,000		225,000
Farmer Mac [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	42,000		42,000		42,000
Norwalk Community Development Corp [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	$ 2,000		$ 2,000		$ 2,000

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