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, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	$ (297,843)	$ (105,797)	$ (395,657)
Unrealized holding gains (losses), before tax	(133,702)	150,187	(51,834)
Foreign currency translation adjustment, before tax	(1,323)	(4,039)	546
Unrealized Gains (losses) on cash flow hedges, before tax	1,453	(165)
Current year benefit plans gains (losses), before tax	(24,200)	(347,993)	260,452
Total other comprehensive income (loss) before reclassifications, before tax	(157,772)	(202,010)	209,164
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities	3,514	3,374	4,287
OTTI charges recognized in net income, before tax			9,800
OTTI charges recognized in net income, tax			(3,847)
OTTI charges recognized in net income, net of tax			5,953
Accretion (Amortization) of net gain (losses) on terminated cash flow hedges, before tax	(141)	7
Losses (gains) realized in net income, before tax	130		33,088
Amortization of prior service credit, before tax	(7,364)	(7,911)	(7,915)
Amortization of actuarial losses, before tax	44,931	14,494	41,436
Total reclassifications, before tax	41,070	9,964	80,696
Total gain (loss) during the period, before tax	(116,702)	(192,046)	289,860
Beginning balance, tax	116,849	41,638	155,393
Ending balance, before tax	(414,545)	(297,843)	(105,797)
Unrealized holding gains (losses), tax	52,376	(58,962)	20,311
Foreign currency translation adjustment, tax	398	1,432	(165)
Unrealized Gains (losses) on cash flow hedges, tax	(572)	65
Current year benefit plans gains (losses), tax	8,612	136,587	(102,227)
Total other comprehensive income (loss) before reclassifications, tax	60,814	79,122	(82,081)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities	(1,383)	(1,324)	(1,683)
Accretion (Amortization) of net gain (losses) on terminated cash flow hedges, tax	56	(3)
Losses (gains) realized in net income, tax	(49)		(12,987)
Amortization of prior service credit, tax	2,620	3,105	3,107
Amortization of actuarial losses, tax	(15,989)	(5,689)	(16,264)
Total reclassifications, tax	(14,745)	(3,911)	(31,674)
Total gain (loss) during the period, tax	46,069	75,211	(113,755)
Beginning balance	(180,994)	(64,159)	(240,264)
Ending balance, tax	162,918	116,849	41,638
Unrealized holding gains (losses), net of tax	(81,326)	91,225	(31,523)
Foreign currency translation adjustment, net of tax	(925)	(2,607)	381
Unrealized Gains (losses) on cash flow hedges	881	(100)
Current year benefit plans gains (losses), net of tax	(15,588)	(211,406)	158,225
Total other comprehensive income (loss) before reclassifications, net of tax	(96,958)	(122,888)	127,083
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities	2,131	2,050	2,604
Accretion (Amortization) of net gain (losses) on terminated cash flow hedges, net of tax	(85)	4
Losses (gains) realized in net income, net of tax	81		20,101
Amortization of prior service credit, net of tax	(4,744)	(4,806)	(4,808)
Amortization of actuarial losses, net of tax	28,942	8,805	25,172
Total reclassifications, net of tax	26,325	6,053	49,022
Total gain (loss) during the period, net of tax	(70,633)	(116,835)	176,105
Ending balance	(251,627)	(180,994)	(64,159)
Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	7,438	37,255	(91,835)
Unrealized holding gains (losses), before tax	8,921	(29,818)	77,794
Total other comprehensive income (loss) before reclassifications, before tax	8,921	(29,818)	77,794
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities		1	279
OTTI charges recognized in net income, before tax			9,800
Losses (gains) realized in net income, before tax			41,217
Total reclassifications, before tax		1	51,296
Total gain (loss) during the period, before tax	8,921	(29,817)	129,090
Ending balance, before tax	16,359	7,438	37,255
Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	201,828	18,450	152,199
Unrealized holding gains (losses), before tax	(142,623)	180,005	(129,628)
Total other comprehensive income (loss) before reclassifications, before tax	(142,623)	180,005	(129,628)
Amortization of unrealized holding losses on held-to-maturity ("HTM") securities	3,514	3,373	4,008
Losses (gains) realized in net income, before tax	130		(8,129)
Total reclassifications, before tax	3,644	3,373	(4,121)
Total gain (loss) during the period, before tax	(138,979)	183,378	(133,749)
Ending balance, before tax	62,849	201,828	18,450
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(503,027)	(161,617)	(455,590)
Current year benefit plans gains (losses), before tax	(24,200)	(347,993)	260,452
Total other comprehensive income (loss) before reclassifications, before tax	(24,200)	(347,993)	260,452
Amortization of prior service credit, before tax	(7,364)	(7,911)	(7,915)
Amortization of actuarial losses, before tax	44,931	14,494	41,436
Total reclassifications, before tax	37,567	6,583	33,521
Total gain (loss) during the period, before tax	13,367	(341,410)	293,973
Ending balance, before tax	(489,660)	(503,027)	(161,617)
Beginning balance	(305,589)	(98,182)	(276,771)
Total gain (loss) during the period, net of tax	8,610	(207,407)	178,589
Ending balance	(296,979)	(305,589)	(98,182)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, before tax	(4,082)	115	(431)
Foreign currency translation adjustment, before tax	(1,323)	(4,039)	546
Unrealized Gains (losses) on cash flow hedges, before tax	1,453	(165)
Total other comprehensive income (loss) before reclassifications, before tax	130	(4,204)	546
Accretion (Amortization) of net gain (losses) on terminated cash flow hedges, before tax	(141)	7
Total reclassifications, before tax	(141)	7
Total gain (loss) during the period, before tax	(11)	(4,197)	546
Ending balance, before tax	$ (4,093)	$ (4,082)	$ 115