Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 90,742	$ (14,563)	$ (11,441)	$ 141,069
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	0	0	0	(521)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	0	0	14
Total securities available for sale and transferred securities, before tax amount	90,742	(14,563)	(11,441)	140,562
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	209	309	418	619
Total defined-benefit post-retirement benefit plans	209	309	418	619
Total other comprehensive income (loss), before tax	90,951	(14,254)	(11,023)	141,181
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	19,056	(3,058)	(2,402)	29,625
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	0	0	0	(109)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	0	0	3
Total securities available for sale and transferred securities, Tax Expense (Benefit)	19,056	(3,058)	(2,402)	29,519
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	44	65	88	130
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	44	65	88	130
Total other comprehensive income (loss), Tax Expense (Benefit)	19,100	(2,993)	(2,314)	29,649
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	71,686	(11,505)	(9,039)	111,444
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	0	0	0	(412)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	0	0	11
Total securities available for sale and transferred securities, Net of Tax	71,686	(11,505)	(9,039)	111,043
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	165	244	330	489
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	165	244	330	489
Other comprehensive income (loss), net of tax	$ 71,851	$ (11,261)	$ (8,709)	$ 111,532