Other Comprehensive Income (Loss) - Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Equity [Abstract]
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Before Tax Amount	$ (113,268)	$ 45,112	$ (78,741)	$ 66,543
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Before Tax Amount	(8,207)	(9,175)	(16,094)	(18,373)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Before Tax Amount	0	(2)	(228)	(2)
Total securities available for sale and transferred securities, Before Tax Amount	(121,475)	35,935	(95,063)	48,168
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Before Tax Amount	1,749	671	3,498	1,343
Derivatives, Reclassification adjustments for gains on interest rate swaps on variable-rate loans included in net income, Before Tax Amount	0	(9,345)	0	(18,690)
Total other comprehensive income (loss), before tax	(119,726)	27,261	(91,565)	30,821
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Tax Expense (Benefit)	(39,643)	15,789	(27,559)	23,290
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Tax Expense (Benefit)	(2,873)	(3,212)	(5,633)	(6,431)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Tax Expense (Benefit)	0	0	(80)	0
Total securities available for sale and transferred securities, Tax Expense (Benefit)	(42,516)	12,577	(33,272)	16,859
Defined-benefit post-retirement benefit plans, Change in the net actuarial gain/loss, Tax Expense (Benefit)	612	235	1,224	470
Derivatives, Reclassification adjustments for gains on interest rate swaps on variable-rate loans included in net income, Tax Expense (Benefit)	0	(3,271)	0	(6,542)
Total other comprehensive income (loss), Tax Expense (Benefit)	(41,904)	9,541	(32,048)	10,787
Securities available for sale and transferred securities, Change in net unrealized gain/loss during the period, Net of Tax	(73,625)	29,323	(51,182)	43,253
Securities available for sale and transferred securities, Change in net unrealized gain on securities transferred to held to maturity, Net of Tax	(5,334)	(5,963)	(10,461)	(11,942)
Securities available for sale and transferred securities, Reclassification adjustment for net (gains) losses included in net income, Net of Tax	0	(2)	(148)	(2)
Total securities available for sale and transferred securities, Net of Tax	(78,959)	23,358	(61,791)	31,309
Defined-benefit post-retirement benefit plans,Change in the net actuarial gain/loss, Net of Tax	1,137	436	2,274	873
Derivatives, Reclassification adjustments for gains on interest rate swaps on variabl-rate loans included in net income, Net of Tax	0	(6,074)	0	(12,148)
Other comprehensive income (loss), net of tax	$ (77,822)	$ 17,720	$ (59,517)	$ 20,034