Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 90,390	$ 165,288	$ 124,201	$ 287,506
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(3,860)	(9,185)	(10,146)	(17,351)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	50	0	50	(14,903)
Total securities available for sale and transferred securities, before tax amount	86,580	156,103	114,105	255,252
Change in the net actuarial gain/loss,Before Tax Amount	0	(862)	0	(862)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, before tax amount	1,358	1,740	2,715	3,293
Total defined-benefit post-retirement benefit plans	1,358	878	2,715	2,431
Total other comprehensive income (loss), before tax	87,938	156,981	116,820	257,683
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	31,636	57,851	43,470	100,627
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(1,351)	(3,215)	(3,551)	(6,073)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	18	0	18	(5,216)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	30,303	54,636	39,937	89,338
Change in the net actuarial gain/loss,Tax Expense (Benefit)	0	(302)	0	(302)
Reclassification adjustment for net amortization of actuarial gain/loss included in net income, Tax Expense (Benefit)	475	609	950	1,153
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	475	307	950	851
Total other comprehensive income (loss), Tax Expense (Benefit)	30,778	54,943	40,887	90,189
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	58,754	107,437	80,731	186,879
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(2,509)	(5,970)	(6,595)	(11,278)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	32	0	32	(9,687)
Total securities available for sale and transferred securities, Net of Tax	56,277	101,467	74,168	165,914
Change in the net actuarial gain/loss, Net of Tax	0	(560)	0	(560)
Reclassification adjustment for net amortization of actuarial gain/lloss included in net income, Net of Tax	883	1,131	1,765	2,140
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	883	571	1,765	1,580
Other comprehensive income (loss), net of tax	$ 57,160	$ 102,038	$ 75,933	$ 167,494