Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (11,884)	$ 90,390	$ (190,788)	$ 124,201
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(2,041)	(3,860)	(4,660)	(10,146)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	60	50	79	50
Total securities available for sale and transferred securities, before tax amount	(13,865)	86,580	(195,369)	114,105
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	1,251	1,358	2,501	2,715
Total defined-benefit post-retirement benefit plans	1,251	1,358	2,501	2,715
Total other comprehensive income (loss), before tax	(12,614)	87,938	(192,868)	116,820
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(2,496)	31,636	(40,066)	43,470
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(429)	(1,351)	(979)	(3,551)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	13	18	17	18
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(2,912)	30,303	(41,028)	39,937
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)	263	475	526	950
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	263	475	526	950
Total other comprehensive income (loss), Tax Expense (Benefit)	(2,649)	30,778	(40,502)	40,887
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(9,388)	58,754	(150,722)	80,731
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(1,612)	(2,509)	(3,681)	(6,595)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	47	32	62	32
Total securities available for sale and transferred securities, Net of Tax	(10,953)	56,277	(154,341)	74,168
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	883	1,975	1,765
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	988	883	1,975	1,765
Other comprehensive income (loss), net of tax	$ (9,965)	$ 57,160	$ (152,366)	$ 75,933