Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 20,332	$ 107,786	$ 406,959	$ 464,072
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(294)	(305)	(979)	(957)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(97)	(108,989)	(266)
Total securities available for sale and transferred securities, before tax amount	20,038	107,384	296,991	462,849
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,330	1,406	3,989	4,218
Total defined-benefit post-retirement benefit plans	1,330	1,406	3,989	4,218
Total other comprehensive income (loss), before tax	21,368	108,790	300,980	467,067
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	4,269	22,635	85,462	97,455
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(62)	(64)	(206)	(201)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(21)	(22,888)	(56)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	4,207	22,550	62,368	97,198
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	280	295	837	886
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	280	295	837	886
Total other comprehensive income (loss), Tax Expense (Benefit)	4,487	22,845	63,205	98,084
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	16,063	85,151	321,497	366,617
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(232)	(241)	(773)	(756)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(76)	(86,101)	(210)
Total securities available for sale and transferred securities, Net of Tax	15,831	84,834	234,623	365,651
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,050	1,111	3,152	3,332
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,050	1,111	3,152	3,332
Other comprehensive income (loss), net of tax	$ 16,881	$ 85,945	$ 237,775	$ 368,983