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, 2018	Mar. 31, 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	(145,457)	(3,015)
Foreign currency translation adjustment, before tax	1,632	732
Unrealized losses on cash flow hedges, before tax	(14,719)
Total other comprehensive income (loss) before reclassifications, before tax	(158,544)	(2,283)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	810	787
Accretion of net gain (losses) on terminated cash flow hedges, before tax	(28)	(39)
Net yield adjustment from cash flow hedges currently in effect, before tax	580
Amortization of prior service credit, before tax	(1,050)	(225)
Amortization of actuarial losses, before tax	10,900	6,775
Total other comprehensive income (loss), before tax	(147,332)	5,015
Ending balance, before tax	(663,417)	(480,230)
Beginning balance, tax	129,476	190,609
Cumulative effect of change in accounting principles — equity securities, tax	5,942
Unrealized holding gains (losses), tax	44,176	1,182
Foreign currency translation adjustment, tax	(342)	(256)
Unrealized losses on cash flow hedges, tax	3,870
Total other comprehensive income (loss) before reclassifications, tax	47,704	926
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, tax	(213)	(310)
Accretion of net gain (losses) on terminated cash flow hedges, tax	7	16
Net yield adjustment from cash flow hedges currently in effect, tax	(152)
Amortization of prior service credit, tax	276	88
Amortization of actuarial losses, tax	(2,866)	(2,666)
Total other comprehensive income (loss), tax	44,756	(1,946)
Ending balance, tax	180,174	188,663
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	(101,281)	(1,833)
Foreign currency translation adjustment, net of tax	1,290	476
Unrealized losses on cash flow hedges, net of tax	(10,849)
Total other comprehensive income (loss) before reclassifications, net of tax	(110,840)	(1,357)
Amortization of unrealized holding losses on held-to-maturity ('HTM') securities, net of tax	597	477
Accretion of net gain on terminated cash flow hedges, net of tax	(21)	(23)
Net yield adjustment from cash flow hedges currently in effect, net of tax	428
Amortization of prior service credit, net of tax	(774)	(137)
Amortization of actuarial losses, net of tax	8,034	4,109
Total other comprehensive income (loss)	(102,576)	3,069
Ending balance, net of tax	(483,243)	(291,567)
Total other comprehensive income(loss), before tax	(147,332)	5,015
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	(145,457)
Total other comprehensive income (loss) before reclassifications, before tax	(145,457)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	810
Total other comprehensive income (loss), before tax	(144,647)
Ending balance, before tax	(227,399)
Cumulative effect of change in accounting principles- equity securities	(16,853)
Total other comprehensive income (loss)	(100,684)
Total other comprehensive income(loss), before tax	(144,647)
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	(1,050)	(225)
Amortization of actuarial losses, before tax	10,900	6,775
Total other comprehensive income (loss), before tax	9,850	6,550
Ending balance, before tax	(403,318)	(443,367)
Total other comprehensive income (loss)	7,260
Total other comprehensive income(loss), before tax	9,850	6,550
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(20,165)	(8,268)
Foreign currency translation adjustment, before tax	1,632	732
Unrealized losses on cash flow hedges, before tax	(14,719)
Total other comprehensive income (loss) before reclassifications, before tax	(13,087)	732
Accretion of net gain (losses) on terminated cash flow hedges, before tax	(28)	(39)
Net yield adjustment from cash flow hedges currently in effect, before tax	580
Total other comprehensive income (loss), before tax	(12,535)	693
Ending balance, before tax	(32,700)	(7,575)
Total other comprehensive income(loss), before tax	$ (12,535)	693
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax		46,725
Unrealized holding gains (losses), before tax		(8,628)
Total other comprehensive income (loss) before reclassifications, before tax		(8,628)
Total other comprehensive income (loss), before tax		(8,628)
Ending balance, before tax		38,097
Total other comprehensive income(loss), before tax		(8,628)
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax		(73,785)
Unrealized holding gains (losses), before tax		5,613
Total other comprehensive income (loss) before reclassifications, before tax		5,613
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities, before tax		787
Total other comprehensive income (loss), before tax		6,400
Ending balance, before tax		(67,385)
Total other comprehensive income(loss), before tax		$ 6,400