Allowance for credit losses - Changes in Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 908,290	$ 902,941	$ 878,022
Provision for credit losses	204,000	270,000	368,000
Consolidation of loan securitization trust			2,752
Net charge-offs
Charge-offs	(230,755)	(309,487)	(412,346)
Recoveries	44,325	44,836	66,513
Net charge-offs	(186,430)	(264,651)	(345,833)
Ending balance	925,860	908,290	902,941
Commercial, Financial, Leasing, etc [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	234,022	212,579	219,170
Provision for credit losses	42,510	66,240	58,438
Net charge-offs
Charge-offs	(41,148)	(55,021)	(91,650)
Recoveries	11,375	10,224	26,621
Net charge-offs	(29,773)	(44,797)	(65,029)
Ending balance	246,759	234,022	212,579
Real Estate Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	367,637	400,562	359,770
Provision for credit losses	5,211	44,404	159,023
Net charge-offs
Charge-offs	(41,945)	(86,869)	(124,087)
Recoveries	6,198	9,540	5,856
Net charge-offs	(35,747)	(77,329)	(118,231)
Ending balance	337,101	367,637	400,562
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	91,915	86,351	91,582
Provision for credit losses	34,864	57,081	52,960
Consolidation of loan securitization trust			2,752
Net charge-offs
Charge-offs	(44,314)	(58,351)	(71,016)
Recoveries	6,342	6,834	10,073
Net charge-offs	(37,972)	(51,517)	(60,943)
Ending balance	88,807	91,915	86,351
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	143,121	133,067	137,124
Provision for credit losses	119,235	101,062	97,573
Net charge-offs
Charge-offs	(103,348)	(109,246)	(125,593)
Recoveries	20,410	18,238	23,963
Net charge-offs	(82,938)	(91,008)	(101,630)
Ending balance	179,418	143,121	133,067
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	71,595	70,382	70,376
Provision for credit losses	2,180	1,213	6
Net charge-offs
Charge-offs
Recoveries
Net charge-offs
Ending balance	$ 73,775	$ 71,595	$ 70,382