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, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	$ (414,545)	$ (297,843)
Unrealized holding gains (losses), before tax	159,290	40,074
Foreign currency translation adjustment, before tax	(83)	(3,732)
Gains on cash flow hedges, before tax		1,453
Total other comprehensive income(loss) before reclassifications, before tax	159,207	37,795
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	968	739
Gains realized in net income, before tax	(4)	98
Accretion of net gain on terminated cash flow hedges, before tax	(39)	(24)
Amortization of prior service credit, before tax	(1,175)	(1,875)
Amortization of actuarial losses, before tax	8,300	11,200
Total reclassifications, before tax	8,050	10,138
Total gain (loss) during the period, before tax	167,257	47,933
Ending balance, before tax	(247,288)	(249,910)
Beginning balance, tax	162,918	116,849
Unrealized holding gains (losses), net of tax	(62,680)	(15,247)
Foreign currency translation adjustment, tax	30	1,348
Gains on cash flow hedges, tax		(568)
Total other comprehensive income(loss) before reclassifications, tax	(62,650)	(14,467)
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities, tax	(381)	(289)
Gains realized in net income, tax	1	(36)
Accretion of net gain on terminated cash flow hedges, tax	15	10
Amortization of prior service credit, tax	462	934
Amortization of actuarial losses, tax	(3,266)	(5,582)
Total reclassifications, tax	(3,169)	(4,963)
Total gain (loss) during the period, tax	(65,819)	(19,430)
Ending balance, tax	97,099	97,419
Beginning balance	(251,627)	(180,994)
Unrealized holding gains (losses), net of tax	96,610	24,827
Foreign currency translation adjustment, net of tax	(53)	(2,384)
Gains on cash flow hedges, net of tax		885
Total other comprehensive income(loss) before reclassifications, net of tax	96,557	23,328
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities, net of tax	587	450
Gains realized in net income, net of tax	(3)	62
Accretion of net gain on terminated cash flow hedges, net of tax	(24)	(14)
Amortization of prior service credit, net of tax	(713)	(941)
Amortization of actuarial losses, net of tax	5,034	5,618
Total reclassifications, net of tax	4,881	5,175
Total other comprehensive income	101,438	28,503
Ending balance	(150,189)	(152,491)
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	16,359	7,438
Unrealized holding gains (losses), before tax	(370)	8,011
Total other comprehensive income(loss) before reclassifications, before tax	(370)	8,011
Total gain (loss) during the period, before tax	(370)	8,011
Ending balance, before tax	15,989	15,449
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	62,849	201,828
Unrealized holding gains (losses), before tax	159,660	32,063
Total other comprehensive income(loss) before reclassifications, before tax	159,660	32,063
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities, before tax	968	739
Gains realized in net income, before tax	(4)	98
Total reclassifications, before tax	964	837
Total gain (loss) during the period, before tax	160,624	32,900
Ending balance, before tax	223,473	234,728
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(489,660)	(503,027)
Amortization of prior service credit, before tax	(1,175)	(1,875)
Amortization of actuarial losses, before tax	8,300	11,200
Total reclassifications, before tax	7,125	9,325
Total gain (loss) during the period, before tax	7,125	9,325
Ending balance, before tax	(482,535)	(493,702)
Beginning balance	(296,979)
Total other comprehensive income	4,321
Ending balance	(292,658)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(4,093)	(4,082)
Foreign currency translation adjustment, before tax	(83)	(3,732)
Gains on cash flow hedges, before tax		1,453
Total other comprehensive income(loss) before reclassifications, before tax	(83)	(2,279)
Accretion of net gain on terminated cash flow hedges, before tax	(39)	(24)
Total reclassifications, before tax	(39)	(24)
Total gain (loss) during the period, before tax	(122)	(2,303)
Ending balance, before tax	$ (4,215)	$ (6,385)