Comprehensive income - Components of Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Available-for-sale ("AFS") investment securities with other- than- temporary impairment ("OTTI"):
Unrealized holding losses, net, before-tax amount			$ (5,251)	$ (35,305)
Less: reclassification adjustment for gains realized in net income, before tax amount				3,814
Less: OTTI charges recognized in net income, before tax amount			(27,976)	(41,713)
Net change for AFS investment securities with OTTI, before tax			22,725	2,594
AFS investment securities - all other:
Unrealized holding gains during period, net, before-tax amount			153,908	147,338
Less: reclassification adjustment for gains realized in net income, before-tax amount			9	146,115
Net change for AFS investment securities - all other, before tax			153,899	1,223
Held- to- maturity ("HTM") investment securities with OTTI:
Unrealized holding gains on HTM securities, net, before-tax amount			(2,848)	(8,500)
Less: reclassification to income of unrealized holding losses, before tax			(1,544)	(213)
Less: OTTI charges recognized in net income, before tax			(5,355)	(10,500)
Net change for HTM investment securities with OTTI, before tax			4,051	2,213
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, before tax			3,495	5,108
Net unrealized gains (losses) on investment securities, before tax			184,170	11,138
Reclassification to income for amortization of gains on terminated cash flow hedges , before tax			(178)	(336)
Foreign currency translation adjustment, before tax			552	(779)
Defined benefit plans liability adjustment, before tax amount			23,368	10,586
Total Comprehensive Income, before-tax amount			207,912	20,609
Unrealized holding losses, net, income taxes			2,061	13,952
Less: reclassification adjustment for gains realized in net income, income taxes				(1,497)
Less: OTTI charges recognized in net income, income taxes			10,980	16,372
Net change for AFS investment securities with OTTI, income taxes			(8,919)	(923)
Unrealized holding gains during period, net, income taxes			(60,362)	(57,629)
Less: reclassification adjustment for gains realized in net income, income taxes			(4)	(57,257)
Net change for AFS investment securities - all other, tax			(60,358)	(372)
Unrealized holding gains on HTM securities, net, income taxes			1,118	3,336
Less: reclassification to income of unrealized holding losses, tax			606	83
Less: OTTI charges recognized in net income, tax			2,102	4,121
Net change for HTM investment securities with OTTI, tax			(1,590)	(868)
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, Tax			(1,372)	(2,005)
Net unrealized gains (losses) on investment securities, income taxes			(72,239)	(4,168)
Reclassification to income for amortization of gains on terminated cash flow hedges, income taxes			66	125
Foreign currency translation adjustment, tax			(201)	276
Defined benefit plans liability adjustment, income taxes			(9,172)	(4,155)
Total Comprehensive Income, income taxes			(81,546)	(7,922)
Unrealized holding losses, net, net of tax amount			(3,190)	(21,353)
Less: reclassification adjustment for gains realized in net income, net				2,317
Less: OTTI charges recognized in net income, net			(16,996)	(25,341)
Net change for AFS investment securities with OTTI, after tax			13,806	1,671
Unrealized holding gains during period, net, net of tax amount			93,546	89,709
Less: reclassification adjustment for gains realized in net income, net			5	88,858
Net change for AFS investment securities - all other, net of tax			93,541	851
Unrealized holding gains on HTM securities, net, net of tax amount			(1,730)	(5,164)
Less: reclassification to income of unrealized holding losses, net of tax			(938)	(130)
Less: OTTI charges recognized in net income, net of tax			(3,253)	(6,379)
Net change for HTM investment securities with OTTI, net of tax			2,461	1,345
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, net of tax			2,123	3,103
Net unrealized gains (losses) on investment securities, net of tax	42,560	34,862	111,931	6,970
Reclassification to income for amortization of gains on terminated cash flow hedges		(70)	(112)	(211)
Foreign currency translation adjustment, net of tax	482	(699)	351	(503)
Defined benefit plans liability adjustment	4,732	2,143	14,196	6,431
Total other comprehensive income	$ 47,774	$ 36,236	$ 126,366	$ 12,687