Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Realized gain (loss) on debt securities andTotal amounts reclassified out of accumulated other comprehensive income (loss)	$ 2,124	$ (2)	$ 3,266	$ 2,945
Unrealized holding gains (losses) on available for sale securities, net of tax	16,399	44,538	47,505	36,192
Amortization of actuarial losses and Total amounts reclassified out of accumulated other comprehensive loss	(165)	(114)	(493)	(344)
Total other comprehensive income	16,399	44,538	47,505	36,192
Unrealized holding gains on available for sale securities before reclassifications
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains on available for sale securities before reclassifications	21,158	63,232	64,177	48,438
Realized gain (loss) on debt securities andTotal amounts reclassified out of accumulated other comprehensive income (loss)	2,124	(2)	3,266	2,945
Unrealized holding gains (losses) on available for sale securities after reclassifications	23,282	63,230	67,443	51,383
Tax effect	(6,883)	(18,692)	(19,938)	(15,191)
Unrealized holding gains (losses) on available for sale securities, net of tax	16,399	44,538	47,505	36,192
Unfunded status of the supplemental retirement plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unfunded status of the supplemental retirement plans before reclassifications	165	114	493	344
Amortization of actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of actuarial losses and Total amounts reclassified out of accumulated other comprehensive loss	$ (165)	$ (114)	$ (493)	$ (344)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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