Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 107,786	$ (96,147)	$ 464,072	$ (286,935)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(305)	(3,764)	(957)	(8,424)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(97)	34	(266)	113
Total securities available for sale and transferred securities, before tax amount	107,384	(99,877)	462,849	(295,246)
Change in the net actuarial gain/loss,Before Tax Amount	0	0	0	0
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), before tax amount	1,406	1,250	4,218	3,751
Total defined-benefit post-retirement benefit plans	1,406	1,250	4,218	3,751
Total other comprehensive income (loss), before tax	108,790	(98,627)	467,067	(291,495)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	22,635	(20,190)	97,455	(60,256)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(64)	(790)	(201)	(1,769)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(21)	7	(56)	24
Total securities available for sale and transferred securities, Tax Expense (Benefit)	22,550	(20,973)	97,198	(62,001)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	295	262	886	788
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	295	262	886	788
Total other comprehensive income (loss), Tax Expense (Benefit)	22,845	(20,711)	98,084	(61,213)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	85,151	(75,957)	366,617	(226,679)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(241)	(2,974)	(756)	(6,655)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(76)	27	(210)	89
Total securities available for sale and transferred securities, Net of Tax	84,834	(78,904)	365,651	(233,245)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,111	988	3,332	2,963
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,111	988	3,332	2,963
Other comprehensive income (loss), net of tax	$ 85,945	$ (77,916)	$ 368,983	$ (230,282)

Other Comprehensive Income Loss Transfers From Available For Sale To Held To Maturity Securities Net Of Tax

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Other Comprehensive Income (Loss), Transfers from Available-for-sale to Held-to-maturity Securities, before Tax

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Other Comprehensive Income Reclassification Adjustment For Available For Sale Securities Transferred To Held To Maturity Tax

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