Comprehensive Income - Components of Other Comprehensive Income (Loss) and Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	$ (279,959)	$ (569,328)	$ (493,290)
Cumulative effect of change in accounting principle —equity securities			(22,795)
Unrealized holding losses, net, before tax	141,081	247,411	(121,589)
Foreign currency translation adjustment, before tax	2,724	1,381	(2,817)
Unrealized losses on cash flow hedges, before tax	505,042	160,373	(4,965)
Current year benefit plans gains, before tax	(238,218)	(126,618)	19,871
Total other comprehensive income (loss) before reclassifications, before tax	410,629	282,547	(109,500)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	3,606	3,394	4,252
Gains (Loss) realized in net income	(2)	3	(18)
Accretion of net gain (losses) on terminated cash flow hedges, before tax	(125)	(136)	(111)
Net yield adjustment from cash flow hedges currently in effect, before tax	(271,971)	(13,011)	13,339
Amortization of prior service credit, before tax	(4,181)	(4,173)	(4,172)
Amortization of actuarial losses, before tax	56,860	20,745	42,967
Total other comprehensive income (loss), before tax	194,816	289,369	(53,243)
Ending balance, before tax	(85,143)	(279,959)	(569,328)
Beginning balance, tax	73,279	149,247	129,476
Cumulative effect of change in accounting principle —equity securities			5,942
Unrealized holding gains (losses), net, tax	(36,498)	(65,009)	31,946
Foreign currency translation adjustment, tax	(440)	(290)	592
Unrealized losses on cash flow hedges, tax	(130,432)	(42,163)	1,306
Current year benefit plans gains, tax	60,208	33,287	(5,224)
Total other comprehensive income (loss), tax	(107,162)	(74,175)	28,620
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax	(966)	(892)	(1,118)
Gains realized in net income, tax	1	(1)	4
Accretion of net gain on terminated cash flow hedges, tax	34	36	29
Net yield adjustment from cash flow hedges currently in effect, tax	70,239	3,421	(3,507)
Amortization of prior service credit, tax	1,057	1,097	1,097
Amortization of actuarial losses, tax	(14,371)	(5,454)	(11,296)
Total other comprehensive income (loss), tax	(51,168)	(75,968)	13,829
Ending balance, tax	22,111	73,279	149,247
Beginning balance, net of tax	(206,680)	(420,081)	(363,814)
Cumulative effect of change in accounting principle — equity securities			(16,853)
Unrealized holding gains (losses), net of tax	104,583	182,402	(89,643)
Foreign currency translation adjustments	2,284	1,091	(2,225)
Unrealized losses on cash flow hedges, net of tax	374,610	118,210	(3,659)
Current year benefit plans gains, net of tax	(178,010)	(93,331)	14,647
Total other comprehensive income (loss) before reclassifications, net of tax	303,467	208,372	(80,880)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax	2,640	2,502	3,134
Gains realized in net income, net of tax	(1)	2	(14)
Accretion of net gain on terminated cash flow hedges, net of tax	(91)	(100)	(82)
Net yield adjustment from cash flow hedges currently in effect, net of tax	(201,732)	(9,590)	9,832
Amortization of prior service credit, net of tax	(3,124)	(3,076)	(3,075)
Amortization of actuarial losses, net of tax	42,489	15,291	31,671
Total other comprehensive income (loss)	143,648	213,401	(39,414)
Ending balance, net of tax	(63,032)	(206,680)	(420,081)
Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	50,701	(200,107)
Unrealized holding losses, net, before tax	141,081	247,411
Total other comprehensive income (loss) before reclassifications, before tax	141,081	247,411
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	3,606	3,394
Gains (Loss) realized in net income	(2)	3
Total other comprehensive income (loss), before tax	144,685	250,808
Ending balance, before tax	195,386	50,701	(200,107)
Cumulative effect of change in accounting principle — equity securities			(16,853)
Total other comprehensive income (loss)	107,222	184,906	(86,523)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(464,548)	(354,502)	(413,168)
Current year benefit plans gains, before tax	(238,218)	(126,618)	19,871
Total other comprehensive income (loss) before reclassifications, before tax	(238,218)	(126,618)	19,871
Amortization of prior service credit, before tax	(4,181)	(4,173)	(4,172)
Amortization of actuarial losses, before tax	56,860	20,745	42,967
Total other comprehensive income (loss), before tax	(185,539)	(110,046)	58,666
Ending balance, before tax	(650,087)	(464,548)	(354,502)
Total other comprehensive income (loss)	(138,645)	(81,116)	43,243
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	133,888	(14,719)	(20,165)
Foreign currency translation adjustment, before tax	2,724	1,381	(2,817)
Unrealized losses on cash flow hedges, before tax	505,042	160,373	(4,965)
Total other comprehensive income (loss) before reclassifications, before tax	507,766	161,754	(7,782)
Accretion of net gain (losses) on terminated cash flow hedges, before tax	(125)	(136)	(111)
Net yield adjustment from cash flow hedges currently in effect, before tax	(271,971)	(13,011)	13,339
Total other comprehensive income (loss), before tax	235,670	148,607	5,446
Ending balance, before tax	$ 369,558	133,888	(14,719)
Accumulated Net Unrealized Investment Gain Loss All Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax		$ (200,107)	(59,957)
Cumulative effect of change in accounting principle —equity securities			(22,795)
Unrealized holding losses, net, before tax			(121,589)
Total other comprehensive income (loss) before reclassifications, before tax			(121,589)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			4,252
Gains (Loss) realized in net income			(18)
Total other comprehensive income (loss), before tax			(117,355)
Ending balance, before tax			$ (200,107)