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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			$ (493,290)	$ (485,245)
Cumulative effect of change in accounting principles — equity securities, before tax			(22,795)
Unrealized holding gains (losses), before tax			(230,135)	53,221
Foreign currency translation adjustment, before tax			(1,783)	3,829
Unrealized losses on cash flow hedges, before tax			(26,522)	(1,076)
Total other comprehensive income (loss) before reclassifications, before tax			(258,440)	55,974
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			2,519	2,535
(Gains) losses realized in net income, before tax			(18)	17
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(83)	(109)
Net yield adjustment from cash flow hedges currently in effect, before tax			7,650	(2,275)
Amortization of prior service credit, before tax			(3,129)	(601)
Amortization of actuarial losses, before tax			32,225	21,206
Total other comprehensive income (loss), before tax			(219,276)	76,747
Ending balance, before tax	$ (735,361)	$ (408,498)	(735,361)	(408,498)
Beginning balance, tax			129,476	190,609
Cumulative effect of change in accounting principles — equity securities, tax			5,942
Unrealized holding gains (losses), tax			60,494	(20,934)
Foreign currency translation adjustment, tax			374	(1,340)
Unrealized losses on cash flow hedges, tax			6,972	424
Total other comprehensive income (loss) before reclassifications, tax			67,840	(21,850)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax			(662)	(998)
(Gains) losses realized in net income			4	(7)
Accretion of net gain (losses) on terminated cash flow hedges, tax			22	43
Net yield adjustment from cash flow hedges currently in effect, tax			(2,011)	895
Amortization of prior service credit, tax			823	236
Amortization of actuarial losses, tax			(8,472)	(8,345)
Total other comprehensive income (loss), tax			57,544	(30,026)
Ending balance, tax	192,962	160,583	192,962	160,583
Beginning balance, net of tax			(363,814)	(294,636)
Cumulative effect of change in accounting principles- equity securities			(16,853)
Unrealized holding gains (losses), net of tax			(169,641)	32,287
Foreign currency translation adjustment, net of tax	(265)	863	(1,409)	2,489
Unrealized losses on cash flow hedges, net of tax			(19,550)	(652)
Total other comprehensive income (loss) before reclassifications, net of tax			(190,600)	34,124
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax			1,857	1,537
(Gains) losses realized in net income, net of tax			(14)	10
Accretion of net gain on terminated cash flow hedges, net of tax			(61)	(66)
Net yield adjustment from cash flow hedges currently in effect, net of tax			5,639	(1,380)
Amortization of prior service credit, net of tax			(2,306)	(365)
Amortization of actuarial losses, net of tax			23,753	12,861
Total other comprehensive income (loss)	(24,458)	22,166	(161,732)	46,721
Ending balance, net of tax	(542,399)	(247,915)	(542,399)	(247,915)
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(59,957)
Cumulative effect of change in accounting principles — equity securities, before tax			(22,795)
Unrealized holding gains (losses), before tax			(230,135)
Total other comprehensive income (loss) before reclassifications, before tax			(230,135)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			2,519
(Gains) losses realized in net income, before tax			(18)
Total other comprehensive income (loss), before tax			(227,634)
Ending balance, before tax	(310,386)		(310,386)
Cumulative effect of change in accounting principles- equity securities			(16,853)
Total other comprehensive income (loss)			(167,798)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(413,168)	(449,917)
Amortization of prior service credit, before tax			(3,129)	(601)
Amortization of actuarial losses, before tax			32,225	21,206
Total other comprehensive income (loss), before tax			29,096	20,605
Ending balance, before tax	(384,072)	(429,312)	(384,072)	(429,312)
Total other comprehensive income (loss)			21,447
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(20,165)	(8,268)
Foreign currency translation adjustment, before tax			(1,783)	3,829
Unrealized losses on cash flow hedges, before tax			(26,522)	(1,076)
Total other comprehensive income (loss) before reclassifications, before tax			(28,305)	2,753
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(83)	(109)
Net yield adjustment from cash flow hedges currently in effect, before tax			7,650	(2,275)
Total other comprehensive income (loss), before tax			(20,738)	369
Ending balance, before tax	$ (40,903)	(7,899)	$ (40,903)	(7,899)
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax				46,725
Unrealized holding gains (losses), before tax				(7,365)
Total other comprehensive income (loss) before reclassifications, before tax				(7,365)
(Gains) losses realized in net income, before tax				(50)
Total other comprehensive income (loss), before tax				(7,415)
Ending balance, before tax		39,310		39,310
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax				(73,785)
Unrealized holding gains (losses), before tax				60,586
Total other comprehensive income (loss) before reclassifications, before tax				60,586
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax				2,535
(Gains) losses realized in net income, before tax				67
Total other comprehensive income (loss), before tax				63,188
Ending balance, before tax		$ (10,597)		$ (10,597)