Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ 165,288	$ (113,268)	$ 287,506	$ (78,741)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(9,185)	(8,207)	(17,351)	(16,094)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	0	(14,903)	(228)
Total securities available for sale and transferred securities, before tax amount	156,103	(121,475)	255,252	(95,063)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Before Tax Amount	1,553	1,749	3,106	3,498
Remeasurement of projected benefit obligation related to SERP	(862)	0	(862)	0
Reclassification adjustment for SERP settlement costs included in net income, before tax amount	187	0	187	0
Total defined-benefit post-retirement benefit plans	878	1,749	2,431	3,498
Total other comprehensive income (loss), before tax	156,981	(119,726)	257,683	(91,565)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	57,851	(39,643)	100,627	(27,559)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(3,215)	(2,873)	(6,073)	(5,633)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	0	(5,216)	(80)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	54,636	(42,516)	89,338	(33,272)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Tax Expense (Benefit)	543	612	1,087	1,224
Remeasurement of projected benefit obligation related to SERP, Tax	(302)	0	(302)	0
Reclassification adjustment for SERP settlement costs included in net income, tax expense (benefit) amount	66	0	66	0
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	307	612	851	1,224
Total other comprehensive income (loss), Tax Expense (Benefit)	54,943	(41,904)	90,189	(32,048)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	107,437	(73,625)	186,879	(51,182)
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(5,970)	(5,334)	(11,278)	(10,461)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	0	(9,687)	(148)
Total securities available for sale and transferred securities, Net of Tax	101,467	(78,959)	165,914	(61,791)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/lloss, Net of Tax	1,010	1,137	2,019	2,274
Remeasurement of projected benefit obligation related to SERP, Net of Tax	(560)	0	(560)	0
Reclassification adjustment for SERP settlement costs included in net income, net of tax amount	121	0	121	0
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	571	1,137	1,580	2,274
Other comprehensive income (loss), net of tax	$ 102,038	$ (77,822)	$ 167,494	$ (59,517)