Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 156,509	$ 158,140	$ 386,627	$ 356,286
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(308)	(308)	(685)	(652)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(169)	(108,989)	(169)
Total securities available for sale and transferred securities, before tax amount	156,201	157,663	276,953	355,465
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,329	1,406	2,659	2,812
Total defined-benefit post-retirement benefit plans	1,329	1,406	2,659	2,812
Total other comprehensive income (loss), before tax	157,530	159,069	279,612	358,277
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	32,868	33,209	81,193	74,820
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(65)	(65)	(144)	(137)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	(35)	(22,888)	(35)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	32,803	33,109	58,161	74,648
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	278	296	557	591
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	278	296	557	591
Total other comprehensive income (loss), Tax Expense (Benefit)	33,081	33,405	58,718	75,239
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	123,641	124,931	305,434	281,466
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(243)	(243)	(541)	(515)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(134)	(86,101)	(134)
Total securities available for sale and transferred securities, Net of Tax	123,398	124,554	218,792	280,817
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,051	1,110	2,102	2,221
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,051	1,110	2,102	2,221
Other comprehensive income (loss), net of tax	$ 124,449	$ 125,664	$ 220,894	$ 283,038