Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (96,147)	$ 7,082	$ (286,935)	$ 131,283
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(3,764)	(3,514)	(8,424)	(13,660)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	34	4,867	113	4,917
Total securities available for sale and transferred securities, before tax amount	(99,877)	8,435	(295,246)	122,540
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	1,250	1,357	3,751	4,072
Total defined-benefit post-retirement benefit plans	1,250	1,357	3,751	4,072
Total other comprehensive income (loss), before tax	(98,627)	9,792	(291,495)	126,612
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(20,190)	2,479	(60,256)	45,949
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(790)	(1,230)	(1,769)	(4,781)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	7	1,703	24	1,721
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(20,973)	2,952	(62,001)	42,889
Change in the net actuarial gain/loss,Tax Expense (Benefit)	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), Tax Expense (Benefit)	262	475	788	1,425
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	262	475	788	1,425
Total other comprehensive income (loss), Tax Expense (Benefit)	(20,711)	3,427	(61,213)	44,314
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(75,957)	4,603	(226,679)	85,334
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(2,974)	(2,284)	(6,655)	(8,879)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	27	3,164	89	3,196
Total securities available for sale and transferred securities, Net of Tax	(78,904)	5,483	(233,245)	79,651
Change in the net actuarial gain/loss, Net of Tax	0	0	0	0
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	988	882	2,963	2,647
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	988	882	2,963	2,647
Other comprehensive income (loss), net of tax	$ (77,916)	$ 6,365	$ (230,282)	$ 82,298