Other Comprehensive Income (Loss) (Component of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, before tax amount	$ 514,556	$ (171,662)	$ 277,926
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, before tax amount	(521)	(629)	(649)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, before tax amount	850	96	(66)
Total securities available for sale and transferred securities before tax amount	514,885	(172,195)	277,211
Change in the net actuarial gain/loss before tax amount	1,665	2,439	9,278
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit) before tax amount	1,239	1,673	3,479
Total defined-benefit post-retirement benefit plans	2,904	4,112	12,757
Other comprehensive income (loss), before tax	517,789	(168,083)	289,968
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, tax expense (benefit)	108,056	(36,049)	58,364
Securities available for sale and transferred securities, change in net unrealized gain on securities transfers to held-to-maturity, tax expense (benefit)	(109)	(132)	(136)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, tax expense (benefit)	179	20	(14)
Total securities available for sale and transferred securities, tax expense (benefit)	108,126	(36,161)	58,214
Change in the net actuarial gain/loss, tax expense (benefit)	350	512	1,948
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit) , tax expense (benefit)	260	351	731
Total defined-benefit post-retirement benefit plans, tax expense (benefit)	610	863	2,679
Total other comprehensive income, tax expense (benefit)	108,736	(35,298)	60,893
Securities available for sale and transferred securities, change in net unrealized gain/loss during the period, net of tax amount	406,500	(135,613)	219,562
Securities available for sale and transferred securities. change in net unrealized gain on securities transfers to held-to-maturity, net of tax amount	(412)	(497)	(513)
Securities available for sale and securities transferred, reclassification adjustment for net (gains) losses included in net income, net of tax amount	671	76	(52)
Total securities available for sale and transferred securities, net of tax amount	406,759	(136,034)	218,997
Change in the net actuarial gain/loss, net of tax amount	1,315	1,927	7,330
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a componet of net periodic cost (benefit), net of tax amount	979	1,322	2,748
Total defined-benefit post-retirement benefit plans, net of tax amount	2,294	3,249	10,078
Other comprehensive income (loss), net of tax	$ 409,053	$ (132,785)	$ 229,075