Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (42,111)	$ (206,863)	$ (241,186)	$ 53,406
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(157)	(162)	(316)	(322)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(33)	0	(54)
Total securities available for sale and transferred securities, before tax amount	(42,268)	(207,058)	(241,502)	53,030
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	419	870	837	1,740
Total defined-benefit post-retirement benefit plans	419	870	837	1,740
Total other comprehensive income (loss), before tax	(41,849)	(206,188)	(240,665)	54,770
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(8,843)	(43,441)	(50,649)	11,215
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(33)	(34)	(67)	(67)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(7)	0	(11)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(8,876)	(43,482)	(50,716)	11,137
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	176	366
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	88	183	176	366
Total other comprehensive income (loss), Tax Expense (Benefit)	(8,788)	(43,299)	(50,540)	11,503
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(33,268)	(163,422)	(190,537)	42,191
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(124)	(128)	(249)	(255)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(26)	0	(43)
Total securities available for sale and transferred securities, Net of Tax	(33,392)	(163,576)	(190,786)	41,893
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	331	687	661	1,374
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	331	687	661	1,374
Other comprehensive income (loss), net of tax	$ (33,061)	$ (162,889)	$ (190,125)	$ 43,267