Comprehensive income - Components of Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Available for sale ("AFS") investment securities with other-than-temporary impairment ("OTTI"):
Unrealized holding losses, net, before-tax amount	$ (150)	$ (58,628)	$ (104,039)
Less: reclassification adjustment for gains realized in net income, before-tax amount		3,814
Less: OTTI charges recognized in net income, before tax	(42,467)	(64,919)	(81,199)
Net change for AFS investment securities with OTTI, before tax	42,317	2,477	(22,840)
AFS investment securities - all other:
Unrealized holding gains during period, net, before-tax amount	137,922	144,941	236,167
Less: reclassification adjustment for gains realized in net income, before-tax amount	9	146,115	1,581
Net change for AFS investment securities - all other, before tax	137,913	(1,174)	234,586
Held-to-maturity ("HTM") investment securities with OTTI:
Unrealized holding gains on HTM securities, net, before-tax amount	(2,848)	(10,317)	(11,908)
Less: reclassification to income of unrealized holding losses, before tax	(1,660)	(545)	(7,984)
Less: OTTI charges recognized in net income, before tax	(5,355)	(12,116)	(5,082)
Net change for HTM investment securities with OTTI, before tax	4,167	2,344	1,158
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, before tax	4,530	6,490	8,264
Unrealized holding losses transferred to HTM investment securities with OTTI, before tax		580
Net change for HTM investment securities with OTTI, before tax		7,070
Net unrealized gains (losses) on investment securities, before tax	188,927	10,717	221,168
Reclassification to income for amortization of gains on terminated cash flow hedges , before tax	(178)	(448)	(448)
Foreign currency translation adjustment, before tax	809	(1,240)
Defined benefit plans liability adjustment, before-tax amount	1,555	(258,146)	(6,331)
Total Comprehensive Income, before-tax amount	191,113	(249,117)	214,389
Unrealized holding losses, net, income taxes	59	23,106	40,566
Less: Reclassification adjustment for gains realized in net income, income taxes		(1,497)
Less: OTTI charges recognized in net income, income taxes	16,668	25,480	31,552
Net change for AFS investment securities with OTTI, income taxes	(16,609)	(877)	9,014
Unrealized holding gains during period, net, income taxes	(54,083)	(56,714)	(92,190)
Less: Reclassification adjustment for gains realized in net income, income taxes	(4)	(57,256)	(610)
Net change for AFS investment securities - all other, tax	(54,079)	542	(91,580)
Unrealized holding gains\losses on HTM securities, net, income taxes	1,118	4,049	4,674
Less: reclassification to income of unrealized holding losses, income taxes	652	214	3,134
Less: OTTI charges recognized in net income, income taxes	2,102	4,755	1,995
Net change for HTM investment securities with OTTI, tax	(1,636)	(920)	(455)
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, Tax	(1,778)	(2,548)	(3,243)
Unrealized holding losses transferred to HTM investment securities with OTTI, tax		(227)
Net change for HTM investment securities with OTTI, Tax		(2,775)
Net unrealized gains (losses) on investment securities, income taxes	(74,102)	(4,030)	(86,264)
Reclassification to income for amortization of gains on terminated cash flow hedges, income taxes	66	167	167
Foreign currency translation adjustment, income taxes	(290)	437
Defined benefit plans liability adjustment, income taxes	(610)	101,322	2,485
Total Comprehensive Income, income taxes	(74,936)	97,896	(83,612)
Unrealized holding losses, net, net of tax amount	(91)	(35,522)	(63,473)
Less: Reclassification adjustment for gains realized in net income, net		2,317
Less: OTTI charges recognized in net income, net	(25,799)	(39,439)	(49,647)
Net change for AFS investment securities with OTTI, after tax	25,708	1,600	(13,826)
Unrealized holding gains during period, net, net of tax amount	83,839	88,227	143,977
Less: Reclassification adjustment for gains realized in net income, net	5	88,859	971
Net change for AFS investment securities - all other, after tax	83,834	(632)	143,006
Unrealized holding Losses on HTM securities, net, net of tax amount	(1,730)	(6,268)	(7,234)
Less: reclassification to income of unrealized holding losses, net of tax	(1,008)	(331)	(4,850)
Less: OTTI charges recognized in net income, net of tax	(3,253)	(7,361)	(3,087)
Net change for HTM investment securities with OTTI, net of tax	2,531	1,424	703
Reclassification to income of unrealized holding losses on investment securities previously transferred from AFS to HTM, net of tax	2,752	3,942	5,021
Unrealized holding losses transferred to HTM investment securities with OTTI, net of tax		353
Net change for HTM investment securities with OTTI, net of tax period		4,295
Net unrealized gains (losses) on investment securities, net of tax	114,825	6,687	134,904
Reclassification to income for amortization of gains on terminated cash flow hedges, net of tax	(112)	(281)	(281)
Foreign currency translation adjustment, net of tax	519	(803)
Defined benefit plans liability adjustment, net	945	(156,824)	(3,846)
Total Comprehensive Income, net	$ 116,177	$ (151,221)	$ 130,777