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
	Mar. 31, 2019	Mar. 31, 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	113,805	(145,457)
Foreign currency translation adjustment, before tax	348	1,632
Unrealized gains (losses) on cash flow hedges, before tax	54,347	(14,719)
Total other comprehensive income (loss) before reclassifications, before tax	168,500	(158,544)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	928	810
Losses realized in net income, before tax	7
Accretion of net gain on terminated cash flow hedges, before tax	(28)	(28)
Net yield adjustment from cash flow hedges currently in effect, before tax	6,625	580
Amortization of prior service credit, before tax	(1,050)	(1,050)
Amortization of actuarial losses, before tax	4,150	10,900
Total other comprehensive income (loss), before tax	179,132	(147,332)
Ending balance, before tax	(390,196)	(663,417)
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	(29,902)	44,176
Foreign currency translation adjustment, tax	(73)	(342)
Unrealized gains (losses) on cash flow hedges, tax	(14,288)	3,870
Total other comprehensive income (loss) before reclassifications, tax	(44,263)	47,704
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax	(245)	(213)
Losses realized in net income, tax	(2)
Accretion of net gain on terminated cash flow hedges, tax	7	7
Net yield adjustment from cash flow hedges currently in effect, tax	(1,742)	(152)
Amortization of prior service credit, tax	276	276
Amortization of actuarial losses, tax	(1,091)	(2,866)
Total other comprehensive income (loss), tax	(47,060)	44,756
Ending balance, tax	102,187	180,174
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	83,903	(101,281)
Foreign currency translation adjustment	275	1,290
Unrealized gains (losses) on cash flow hedges, net of tax	40,059	(10,849)
Total other comprehensive income (loss) before reclassifications, net of tax	124,237	(110,840)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax	683	597
Losses realized in net income, net of tax	5
Accretion of net gain on terminated cash flow hedges, net of tax	(21)	(21)
Net yield adjustment from cash flow hedges currently in effect, net of tax	4,883	428
Amortization of prior service credit, net of tax	(774)	(774)
Amortization of actuarial losses, net of tax	3,059	8,034
Total other comprehensive income (loss)	132,072	(102,576)
Ending balance, net of tax	(288,009)	(483,243)
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	113,805	(145,457)
Total other comprehensive income (loss) before reclassifications, before tax	113,805	(145,457)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	928	810
Losses realized in net income, before tax	7
Total other comprehensive income (loss), before tax	114,740	(144,647)
Ending balance, before tax	(85,367)	(227,399)
Total other comprehensive income (loss)	84,591
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	(1,050)	(1,050)
Amortization of actuarial losses, before tax	4,150	10,900
Total other comprehensive income (loss), before tax	3,100	9,850
Ending balance, before tax	(351,402)	(403,318)
Total other comprehensive income (loss)	2,285
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(14,719)	(20,165)
Foreign currency translation adjustment, before tax	348	1,632
Unrealized gains (losses) on cash flow hedges, before tax	54,347	(14,719)
Total other comprehensive income (loss) before reclassifications, before tax	54,695	(13,087)
Accretion of net gain on terminated cash flow hedges, before tax	(28)	(28)
Net yield adjustment from cash flow hedges currently in effect, before tax	6,625	580
Total other comprehensive income (loss), before tax	61,292	(12,535)
Ending balance, before tax	$ 46,573	$ (32,700)