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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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			(187,989)	23,926
Foreign currency translation adjustment, before tax			(1,448)	2,502
Unrealized losses on cash flow hedges, before tax			(21,065)	(441)
Total other comprehensive income (loss) before reclassifications, before tax			(210,502)	25,987
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,590	1,721
(Gains) losses realized in net income, before tax			(18)	17
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(56)	(78)
Net yield adjustment from cash flow hedges currently in effect, before tax			3,469	(1,094)
Amortization of prior service credit, before tax			(2,086)	(400)
Amortization of actuarial losses, before tax			21,483	14,137
Total other comprehensive income (loss), before tax			(186,120)	40,290
Ending balance, before tax	$ (702,205)	$ (444,955)	(702,205)	(444,955)
Beginning balance, tax			129,476	190,609
Cumulative effect of change in accounting principles — equity securities, tax			5,942
Unrealized holding gains (losses), tax			49,414	(9,404)
Foreign currency translation adjustment, tax			304	(876)
Unrealized losses on cash flow hedges, tax			5,538	174
Total other comprehensive income (loss) before reclassifications, tax			55,256	(10,106)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax			(418)	(677)
(Gains) losses realized in net income			4	(7)
Accretion of net gain (losses) on terminated cash flow hedges, tax			15	31
Net yield adjustment from cash flow hedges currently in effect, tax			(912)	430
Amortization of prior service credit, tax			549	157
Amortization of actuarial losses, tax			(5,648)	(5,563)
Total other comprehensive income (loss), tax			48,846	(15,735)
Ending balance, tax	184,264	174,874	184,264	174,874
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			(138,575)	14,522
Foreign currency translation adjustment, net of tax	(2,434)	1,150	(1,144)	1,626
Unrealized losses on cash flow hedges, net of tax			(15,527)	(267)
Total other comprehensive income (loss) before reclassifications, net of tax			(155,246)	15,881
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax			1,172	1,044
(Gains) losses realized in net income, net of tax			(14)	10
Accretion of net gain on terminated cash flow hedges, net of tax			(41)	(47)
Net yield adjustment from cash flow hedges currently in effect, net of tax			2,557	(664)
Amortization of prior service credit, net of tax			(1,537)	(243)
Amortization of actuarial losses, net of tax			15,835	8,574
Total other comprehensive income (loss)	(34,698)	21,486	(137,274)	24,555
Ending balance, net of tax	(517,941)	(270,081)	(517,941)	(270,081)
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			(187,989)
Total other comprehensive income (loss) before reclassifications, before tax			(187,989)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax			1,590
(Gains) losses realized in net income, before tax			(18)
Total other comprehensive income (loss), before tax			(186,417)
Ending balance, before tax	(269,169)		(269,169)
Cumulative effect of change in accounting principles- equity securities			(16,853)
Total other comprehensive income (loss)			(137,417)
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			(2,086)	(400)
Amortization of actuarial losses, before tax			21,483	14,137
Total other comprehensive income (loss), before tax			19,397	13,737
Ending balance, before tax	(393,771)	(436,180)	(393,771)	(436,180)
Total other comprehensive income (loss)			14,298
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax			(20,165)	(8,268)
Foreign currency translation adjustment, before tax			(1,448)	2,502
Unrealized losses on cash flow hedges, before tax			(21,065)	(441)
Total other comprehensive income (loss) before reclassifications, before tax			(22,513)	2,061
Accretion of net gain (losses) on terminated cash flow hedges, before tax			(56)	(78)
Net yield adjustment from cash flow hedges currently in effect, before tax			3,469	(1,094)
Total other comprehensive income (loss), before tax			(19,100)	889
Ending balance, before tax	$ (39,265)	(7,379)	$ (39,265)	(7,379)
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax				46,725
Unrealized holding gains (losses), before tax				(13,802)
Total other comprehensive income (loss) before reclassifications, before tax				(13,802)
(Gains) losses realized in net income, before tax				(50)
Total other comprehensive income (loss), before tax				(13,852)
Ending balance, before tax		32,873		32,873
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax				(73,785)
Unrealized holding gains (losses), before tax				37,728
Total other comprehensive income (loss) before reclassifications, before tax				37,728
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax				1,721
(Gains) losses realized in net income, before tax				67
Total other comprehensive income (loss), before tax				39,516
Ending balance, before tax		$ (34,269)		$ (34,269)