Comprehensive Income - Components of Other Comprehensive Income (Loss) and Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			$ (485,245)	$ (414,545)
Unrealized holding gains (losses), before tax			23,926	236,378
Foreign currency translation adjustment, before tax			2,502	(2,489)
Unrealized losses on cash flow hedges, before tax			(441)
Total other comprehensive income (loss) before reclassifications, before tax			25,987	233,889
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,721	2,081
(Gains) losses realized in net income, before tax			17	(268)
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(78)	(77)
Net yield adjustment from cash flow hedges currently in effect, before tax			(1,094)
Amortization of prior service credit, before tax			(400)	(2,294)
Amortization of actuarial losses, before tax			14,137	15,103
Total reclassifications, before tax			14,303	14,545
Total gain (loss) during the period, before tax			40,290	248,434
Ending balance, before tax	$ (444,955)	$ (166,111)	(444,955)	(166,111)
Beginning balance, tax			190,609	162,918
Unrealized holding gains (losses), tax			(9,404)	(93,010)
Foreign currency translation adjustment, tax			(876)	871
Unrealized losses on cash flow hedges, tax			174
Total other comprehensive income (loss) before reclassifications, tax			(10,106)	(92,139)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, tax			(677)	(819)
(Gains) losses realized in net income, before tax			(7)	102
Accretion of net gain (losses) on terminated cash flow hedges, tax			31	30
Net yield adjustment from cash flow hedges currently in effect, tax			430
Amortization of prior service credit, tax			157	902
Amortization of actuarial losses, tax			(5,563)	(5,904)
Total reclassifications, tax			(5,629)	(5,689)
Total gain (loss) during the period, tax			(15,735)	(97,828)
Ending balance, tax	174,874	65,090	174,874	65,090
Total other comprehensive income (loss) before reclassifications, tax			(10,106)	(92,139)
Beginning balance, net of tax			(294,636)	(251,627)
Unrealized holding gains (losses), net			14,522	143,368
Foreign currency translation adjustment, net of tax	1,150	(1,565)	1,626	(1,618)
Unrealized losses on cash flow hedges, net of tax			(267)
Total other comprehensive income (loss) before reclassifications, net of tax			15,881	141,750
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, net of tax			1,044	1,262
(Gains) losses realized in net income, before tax			10	(166)
Accretion of net gain on terminated cash flow hedges, net of tax			(47)	(47)
Net yield adjustment from cash flow hedges currently in effect, net of tax			(664)
Amortization of prior service credit, net of tax			(243)	(1,392)
Amortization of actuarial losses, net of tax			8,574	9,199
Total reclassifications, net of tax			8,674	8,856
Total other comprehensive income	21,486	49,168	24,555	150,606
Ending balance, net of tax	(270,081)	(101,021)	(270,081)	(101,021)
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			46,725	16,359
Unrealized holding gains (losses), before tax			(13,802)	9,260
Total other comprehensive income (loss) before reclassifications, before tax			(13,802)	9,260
(Gains) losses realized in net income, before tax			(50)
Total reclassifications, before tax			(50)
Total gain (loss) during the period, before tax			(13,852)	9,260
Ending balance, before tax	32,873	25,619	32,873	25,619
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(73,785)	62,849
Unrealized holding gains (losses), before tax			37,728	227,118
Total other comprehensive income (loss) before reclassifications, before tax			37,728	227,118
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,721	2,081
(Gains) losses realized in net income, before tax			67	(268)
Total reclassifications, before tax			1,788	1,813
Total gain (loss) during the period, before tax			39,516	228,931
Ending balance, before tax	(34,269)	291,780	(34,269)	291,780
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(449,917)	(489,660)
Amortization of prior service credit, before tax			(400)	(2,294)
Amortization of actuarial losses, before tax			14,137	15,103
Total reclassifications, before tax			13,737	12,809
Total gain (loss) during the period, before tax			13,737	12,809
Ending balance, before tax	(436,180)	(476,851)	(436,180)	(476,851)
Total other comprehensive income			8,331
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(8,268)	(4,093)
Foreign currency translation adjustment, before tax			2,502	(2,489)
Unrealized losses on cash flow hedges, before tax			(441)
Total other comprehensive income (loss) before reclassifications, before tax			2,061	(2,489)
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(78)	(77)
Net yield adjustment from cash flow hedges currently in effect, before tax			(1,094)
Total reclassifications, before tax			(1,172)	(77)
Total gain (loss) during the period, before tax			889	(2,566)
Ending balance, before tax	$ (7,379)	$ (6,659)	$ (7,379)	$ (6,659)