Securities - Additional Information (Detail)	3 Months Ended
Mar. 31, 2026 USD ($) Security	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($) Security
Debt Securities, Available-for-Sale [Line Items]
Securities available for sale, Sales	$ 0	$ 0
Gross realized gains	0	0
Gross realized losses	0	$ 0
Carrying value of pledged securities	$ 265,791,000		$ 239,649,000
Number of securities in portfolio with unrealized losses \| Security	434		372
Available for sale term	3 years
Equity securities	$ 2,700,000		$ 2,700,000
FHLBC stock	8,905,000		11,645,000
FRBC stock	15,950,000		14,023,000
Other securities	25,144,000		25,942,000
United Bankers Bancorp [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	245,000		230,000
Farmer Mac [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	42,000		42,000
Norwalk Community Development Corp [Member]
Debt Securities, Available-for-Sale [Line Items]
Other securities	$ 2,000		$ 2,000

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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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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