Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (102,183)	$ 155,632
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	0	(521)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	14
Total securities available for sale and transferred securities, before tax amount	(102,183)	155,125
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	310
Total defined-benefit post-retirement benefit plans	209	310
Total other comprehensive income (loss), before tax	(101,974)	155,435
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(21,458)	32,683
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	0	(109)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	3
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(21,458)	32,577
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
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	44	65
Total other comprehensive income (loss), Tax Expense (Benefit)	(21,414)	32,642
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(80,725)	122,949
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	0	(412)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	11
Total securities available for sale and transferred securities, Net of Tax	(80,725)	122,548
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	245
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	165	245
Other comprehensive income (loss), net of tax	$ (80,560)	$ 122,793