Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 230,118	$ 198,146
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(377)	(344)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(108,989)	0
Total securities available for sale and transferred securities, before tax amount	120,752	197,802
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
Total defined-benefit post-retirement benefit plans	1,330	1,406
Total other comprehensive income (loss), before tax	122,082	199,208
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	48,325	41,611
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(79)	(72)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(22,888)	0
Total securities available for sale and transferred securities, Tax Expense (Benefit)	25,358	41,539
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	279	295
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	279	295
Total other comprehensive income (loss), Tax Expense (Benefit)	25,637	41,834
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	181,793	156,535
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(298)	(272)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(86,101)	0
Total securities available for sale and transferred securities, Net of Tax	95,394	156,263
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	1,111
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,051	1,111
Other comprehensive income (loss), net of tax	$ 96,445	$ 157,374