Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (600,441)	$ (829,324)	$ (547,035)	$ (2,376,642)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(164)	(174)	(486)	(572)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(12)	0	(66)	0
Total securities available for sale and transferred securities, before tax amount	(600,617)	(829,498)	(547,587)	(2,377,214)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	869	741	2,609	2,223
Total defined-benefit post-retirement benefit plans	869	741	2,609	2,223
Total other comprehensive income (loss), before tax	(599,748)	(828,757)	(544,978)	(2,374,991)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(126,092)	(174,158)	(114,877)	(499,095)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(35)	(36)	(102)	(120)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(3)	0	(14)	0
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(126,130)	(174,194)	(114,993)	(499,215)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	182	156	548	467
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	182	156	548	467
Total other comprehensive income (loss), Tax Expense (Benefit)	(125,948)	(174,038)	(114,445)	(498,748)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(474,349)	(655,166)	(432,158)	(1,877,547)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(129)	(138)	(384)	(452)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(9)	0	(52)	0
Total securities available for sale and transferred securities, Net of Tax	(474,487)	(655,304)	(432,594)	(1,877,999)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	687	585	2,061	1,756
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	687	585	2,061	1,756
Other comprehensive income (loss), net of tax	$ (473,800)	$ (654,719)	$ (430,533)	$ (1,876,243)