Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (95,641)	$ 86,445	$ 191,865	$ 7,704
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(7,278)	(8,831)	(24,629)	(24,925)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	37	52	(14,866)	(176)
Total securities available for sale and transferred securities, before tax amount	(102,882)	77,666	152,370	(17,397)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Before Tax Amount	1,585	1,748	4,691	5,246
Remeasurement of projected benefit obligation related to SERP	0	0	(862)	0
Reclassification adjustment for SERP settlement costs included in net income, before tax amount	0	0	187	0
Total defined-benefit post-retirement benefit plans	1,585	1,748	4,016	5,246
Total other comprehensive income (loss), before tax	(101,297)	79,414	156,386	(12,151)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(33,473)	30,256	67,154	2,697
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(2,547)	(3,091)	(8,620)	(8,724)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	12	18	(5,204)	(62)
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(36,008)	27,183	53,330	(6,089)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Tax Expense (Benefit)	555	612	1,642	1,836
Remeasurement of projected benefit obligation related to SERP, Tax	0	0	(302)	0
Reclassification adjustment for SERP settlement costs included in net income, tax expense (benefit) amount	0	0	66	0
Total defined-benefit post-retirement benefit plans, Tax Expense (Benefit)	555	612	1,406	1,836
Total other comprehensive income (loss), Tax Expense (Benefit)	(35,453)	27,795	54,736	(4,253)
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(62,168)	56,189	124,711	5,007
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(4,731)	(5,740)	(16,009)	(16,201)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	25	34	(9,662)	(114)
Total securities available for sale and transferred securities, Net of Tax	(66,874)	50,483	99,040	(11,308)
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/lloss, Net of Tax	1,030	1,136	3,049	3,410
Remeasurement of projected benefit obligation related to SERP, Net of Tax	0	0	(560)	0
Reclassification adjustment for SERP settlement costs included in net income, net of tax amount	0	0	121	0
Total defined-benefit post-retirement benefit plans, Net of Tax Amount	1,030	1,136	2,610	3,410
Other comprehensive income (loss), net of tax	$ (65,844)	$ 51,619	$ 101,650	$ (7,898)