Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (960,293)	$ 273,173	$ (971,734)	$ 414,242
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	7,512	0	7,512	14
Total securities available for sale and transferred securities, before tax amount	(952,781)	273,173	(964,222)	413,735
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	207	310	625	929
Total defined-benefit post-retirement benefit plans	207	310	625	929
Total other comprehensive income (loss), before tax	(952,574)	273,483	(963,597)	414,664
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(201,662)	57,366	(204,064)	86,991
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)	1,578	0	1,578	3
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(200,084)	57,366	(202,486)	86,885
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	43	65	131	195
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	43	65	131	195
Total other comprehensive income (loss), Tax Expense (Benefit)	(200,041)	57,431	(202,355)	87,080
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(758,631)	215,807	(767,670)	327,251
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	5,934	0	5,934	11
Total securities available for sale and transferred securities, Net of Tax	(752,697)	215,807	(761,736)	326,850
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	164	245	494	734
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	164	245	494	734
Other comprehensive income (loss), net of tax	$ (752,533)	$ 216,052	$ (761,242)	$ 327,584