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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			$ (569,328)	$ (493,290)
Cumulative effect of change in accounting principles — equity securities, before tax				(22,795)
Unrealized holding gains (losses), net, before tax			207,802	(187,989)
Foreign currency translation adjustment, before tax			(507)	(1,448)
Unrealized gains (losses) on cash flow hedges, before tax			188,272	(21,065)
Total other comprehensive income (loss) before reclassifications, before tax			395,567	(210,502)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,691	1,590
(Gains) losses realized in net income, before tax			7	(18)
Accretion of net gain on terminated cash flow hedges, before tax			(56)	(56)
Net yield adjustment from cash flow hedges currently in effect, before tax			11,176	3,469
Amortization of prior service credit, before tax			(2,086)	(2,086)
Amortization of actuarial losses, before tax			10,373	21,483
Total other comprehensive income (loss), before tax			416,672	(186,120)
Ending balance, before tax	$ (152,656)	$ (702,205)	(152,656)	(702,205)
Beginning balance, tax			149,247	129,476
Cumulative effect of change in accounting principles — equity securities, tax				5,942
Unrealized holding gains (losses), net, tax			(54,609)	49,414
Foreign currency translation adjustment, tax			107	304
Unrealized gains (losses) on cash flow hedges, tax			(49,497)	5,538
Total other comprehensive income (loss) before reclassifications, tax			(103,999)	55,256
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax			(445)	(418)
(Gains) losses realized in net income, tax			(2)	4
Accretion of net gain on terminated cash flow hedges, tax			14	15
Net yield adjustment from cash flow hedges currently in effect, tax			(2,938)	(912)
Amortization of prior service credit, tax			548	549
Amortization of actuarial losses, tax			(2,727)	(5,648)
Total other comprehensive income (loss), tax			(109,549)	48,846
Ending balance, tax	39,698	184,264	39,698	184,264
Beginning balance, net of tax			(420,081)	(363,814)
Cumulative effect of change in accounting principle — equity securities				(16,853)
Unrealized holding gains (losses), net of tax			153,193	(138,575)
Foreign currency translation adjustment	(675)	(2,434)	(400)	(1,144)
Unrealized gains (losses) on cash flow hedges, net of tax			138,775	(15,527)
Total other comprehensive income (loss) before reclassifications, net of tax			291,568	(155,246)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax			1,246	1,172
(Gains) losses realized in net income, net of tax			5	(14)
Accretion of net gain on terminated cash flow hedges, net of tax			(42)	(41)
Net yield adjustment from cash flow hedges currently in effect, net of tax			8,238	2,557
Amortization of prior service credit, net of tax			(1,538)	(1,537)
Amortization of actuarial losses, net of tax			7,646	15,835
Total other comprehensive income (loss)	175,051	(34,698)	307,123	(137,274)
Ending balance, net of tax	(112,958)	(517,941)	(112,958)	(517,941)
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(200,107)	(59,957)
Cumulative effect of change in accounting principles — equity securities, before tax				(22,795)
Unrealized holding gains (losses), net, before tax			207,802	(187,989)
Total other comprehensive income (loss) before reclassifications, before tax			207,802	(187,989)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,691	1,590
(Gains) losses realized in net income, before tax			7	(18)
Total other comprehensive income (loss), before tax			209,500	(186,417)
Ending balance, before tax	9,393	(269,169)	9,393	(269,169)
Total other comprehensive income (loss)			154,444
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(354,502)	(413,168)
Amortization of prior service credit, before tax			(2,086)	(2,086)
Amortization of actuarial losses, before tax			10,373	21,483
Total other comprehensive income (loss), before tax			8,287	19,397
Ending balance, before tax	(346,215)	(393,771)	(346,215)	(393,771)
Total other comprehensive income (loss)			6,108
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(14,719)	(20,165)
Foreign currency translation adjustment, before tax			(507)	(1,448)
Unrealized gains (losses) on cash flow hedges, before tax			188,272	(21,065)
Total other comprehensive income (loss) before reclassifications, before tax			187,765	(22,513)
Accretion of net gain on terminated cash flow hedges, before tax			(56)	(56)
Net yield adjustment from cash flow hedges currently in effect, before tax			11,176	3,469
Total other comprehensive income (loss), before tax			198,885	(19,100)
Ending balance, before tax	$ 184,166	$ (39,265)	$ 184,166	$ (39,265)