Loans and leases and the allowance for credit losses - Summary of Changes in Accretable Yield for Acquired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Credit Derivatives [Line Items]
Balance at beginning of period	$ 788,760	$ 838,765
Interest income	(87,372)	(258,629)
Reclassifications from (to) nonaccretable balance, net	542	140,786
Other	1,382	(17,610)
Balance at end of period	703,312	703,312
Purchase Impaired [Member]
Credit Derivatives [Line Items]
Balance at beginning of period	55,599	30,805
Interest income	(12,436)	(29,721)
Reclassifications from (to) nonaccretable balance, net	542	42,621
Balance at end of period	43,705	43,705
Other [Member]
Credit Derivatives [Line Items]
Balance at beginning of period	733,161	807,960
Interest income	(74,936)	(228,908)
Reclassifications from (to) nonaccretable balance, net		98,165
Other	1,382	(17,610)
Balance at end of period	$ 659,607	$ 659,607