Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 158,140	$ (11,884)	$ 356,286	$ (190,788)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(308)	(2,041)	(652)	(4,660)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	(169)	60	(169)	79
Total securities available for sale and transferred securities, before tax amount	157,663	(13,865)	355,465	(195,369)
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	1,251	2,812	2,501
Total defined-benefit post-retirement benefit plans	1,406	1,251	2,812	2,501
Total other comprehensive income (loss), before tax	159,069	(12,614)	358,277	(192,868)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	33,209	(2,496)	74,820	(40,066)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(65)	(429)	(137)	(979)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	(35)	13	(35)	17
Total securities available for sale and transferred securities, Tax Expense (Benefit)	33,109	(2,912)	74,648	(41,028)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income as a component of net periodic cost (benefit), Tax Expense (Benefit)	296	263	591	526
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	296	263	591	526
Total other comprehensive income (loss), Tax Expense (Benefit)	33,405	(2,649)	75,239	(40,502)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	124,931	(9,388)	281,466	(150,722)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(243)	(1,612)	(515)	(3,681)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	(134)	47	(134)	62
Total securities available for sale and transferred securities, Net of Tax	124,554	(10,953)	280,817	(154,341)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income as a component of net periodic cost (benefit), Net of Tax	1,110	988	2,221	1,975
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,110	988	2,221	1,975
Other comprehensive income (loss), net of tax	$ 125,664	$ (9,965)	$ 283,038	$ (152,366)