Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended
Jun. 30, 2026 USD ($) Security	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($) Security	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($) Security
Debt Securities, Available-for-Sale [Line Items]
Securities available for sale, Sales	$ 0	$ 0	$ 0	$ 0
Gross realized gains	0	0	0	0
Gross realized losses	0	$ 0	0	$ 0
Carrying value of pledged securities	$ 250,182,000		$ 250,182,000		$ 239,649,000
Number of securities in portfolio with unrealized losses \| Security	391		391		372
Available for sale term	4 years 8 months 12 days		4 years 8 months 12 days
Equity securities	$ 2,900,000		$ 2,900,000		$ 2,700,000
FHLBC stock	12,641,000		12,641,000		11,645,000
FRBC stock	16,027,000		16,027,000		14,023,000
Other securities	28,957,000		28,957,000		25,942,000
United Bankers Bancorp [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	245,000		245,000		230,000
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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