Comprehensive income - Components of Other Comprehensive Income (Loss) and Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Components Of Comprehensive Income [Line Items]
Beginning balance	$ (395,657)	$ (586,769)
Unrealized holding gains (losses), net	5,581	19,611
Foreign currency translation adjustment	(1,452)	622
Total other comprehensive income before reclassifications	4,129	20,233
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities	1,195	1,942
OTTI charges recognized in net income	9,800	11,486
Gains realized in net income		(45)
Amortization of gains on terminated cash flow hedges		(112)
Amortization of prior service credit	(2,000)	(1,650)
Amortization of actuarial losses	10,500	9,500
Total reclassifications	19,495	21,121
Total gain (loss) during the period	23,624	41,354
Ending balance	(372,033)	(545,415)
Beginning balance	155,393	230,328
Unrealized holding gains (losses), net	(2,181)	(7,657)
Foreign currency translation adjustment	520	(220)
Total other comprehensive income before reclassifications	(1,661)	(7,877)
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities	(469)	(762)
OTTI charges recognized in net income	(3,847)	(4,508)
Gains realized in net income		15
Amortization of gains on terminated cash flow hedges		42
Amortization of prior service credit	785	648
Amortization of actuarial losses	(4,121)	(3,729)
Total reclassifications	(7,652)	(8,294)
Total gain (loss) during the period	(9,313)	(16,171)
Ending balance	146,080	214,157
Balance - December 31, 2012	(240,264)	(356,441)
Unrealized holding gains (losses), net	3,400	11,954
Foreign currency translation adjustment	(932)	402
Total other comprehensive income before reclassifications	2,468	12,356
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities	726	1,180
OTTI charges recognized in net income	5,953	6,978
Gains realized in net income		(30)
Amortization of gains on terminated cash flow hedges		(70)
Amortization of prior service credit	(1,215)	(1,002)
Amortization of actuarial losses	6,379	5,771
Total reclassifications	11,843	12,827
Total other comprehensive income	14,311	25,183
Balance - March 31, 2013	(225,953)	(331,258)
Investment Securities With OTTI [Member]
Components Of Comprehensive Income [Line Items]
Beginning balance	(91,835)	(138,319)
Unrealized holding gains (losses), net	24,540	(10,085)
Total other comprehensive income before reclassifications	24,540	(10,085)
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities	49	819
OTTI charges recognized in net income	9,800	11,486
Total reclassifications	9,849	12,305
Total gain (loss) during the period	34,389	2,220
Ending balance	(57,446)	(136,099)
Investment Securities All Other [Member]
Components Of Comprehensive Income [Line Items]
Beginning balance	152,199	9,757
Unrealized holding gains (losses), net	(18,959)	29,696
Total other comprehensive income before reclassifications	(18,959)	29,696
Accretion of unrealized holding losses on held-to-maturity ("HTM") securities	1,146	1,123
Gains realized in net income		(45)
Total reclassifications	1,146	1,078
Total gain (loss) during the period	(17,813)	30,774
Ending balance	134,386	40,531
Defined Benefit Plans [Member]
Components Of Comprehensive Income [Line Items]
Beginning balance	(455,590)	(457,145)
Amortization of prior service credit	(2,000)	(1,650)
Amortization of actuarial losses	10,500	9,500
Total reclassifications	8,500	7,850
Total gain (loss) during the period	8,500	7,850
Ending balance	(447,090)	(449,295)
Balance - December 31, 2012	(276,771)
Total other comprehensive income	5,164
Balance - March 31, 2013	(271,607)
Other [Member]
Components Of Comprehensive Income [Line Items]
Beginning balance	(431)	(1,062)
Foreign currency translation adjustment	(1,452)	622
Total other comprehensive income before reclassifications	(1,452)	622
Amortization of gains on terminated cash flow hedges		(112)
Total reclassifications		(112)
Total gain (loss) during the period	(1,452)	510
Ending balance	(1,883)	(552)
Balance - December 31, 2012	(284)
Total other comprehensive income	(932)
Balance - March 31, 2013	$ (1,216)