Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (80,668)	$ 20,332	$ (209,779)	$ 406,959
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(237)	(294)	(741)	(979)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	0	0	(108,989)
Total securities available for sale and transferred securities, before tax amount	(80,905)	20,038	(210,520)	296,991
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,529	1,330	4,587	3,989
Total defined-benefit post-retirement benefit plans	1,529	1,330	4,587	3,989
Total other comprehensive income (loss), before tax	(79,376)	21,368	(205,933)	300,980
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(16,941)	4,269	(44,054)	85,462
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(49)	(62)	(155)	(206)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	0	0	(22,888)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(16,990)	4,207	(44,209)	62,368
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	321	280	963	837
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	321	280	963	837
Total other comprehensive income (loss), Tax Expense (Benefit)	(16,669)	4,487	(43,246)	63,205
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(63,727)	16,063	(165,725)	321,497
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(188)	(232)	(586)	(773)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	0	0	(86,101)
Total securities available for sale and transferred securities, Net of Tax	(63,915)	15,831	(166,311)	234,623
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,208	1,050	3,624	3,152
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,208	1,050	3,624	3,152
Other comprehensive income (loss), net of tax	$ (62,707)	$ 16,881	$ (162,687)	$ 237,775