Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (199,075)	$ 260,269
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(159)	(160)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(21)
Total securities available for sale and transferred securities, before tax amount	(199,234)	260,088
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	418	870
Total defined-benefit post-retirement benefit plans	418	870
Total other comprehensive income (loss), before tax	(198,816)	260,958
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(41,806)	54,656
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(34)	(33)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(4)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(41,840)	54,619
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	88	183
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	88	183
Total other comprehensive income (loss), Tax Expense (Benefit)	(41,752)	54,802
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(157,269)	205,613
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(125)	(127)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(17)
Total securities available for sale and transferred securities, Net of Tax	(157,394)	205,469
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	330	687
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	330	687
Other comprehensive income (loss), net of tax	$ (157,064)	$ 206,156