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
	Mar. 31, 2013	Mar. 31, 2012
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 680,524	$ 838,765
Interest income	(70,451)	(81,387)
Reclassifications from (to) nonaccretable balance, net	10,997	424
Other	(9,733)	12,229
Balance at end of period	611,337	770,031
Purchased impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	42,252	30,805
Interest income	(8,704)	(7,664)
Reclassifications from (to) nonaccretable balance, net	180	(576)
Balance at end of period	33,728	22,565
Other [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	638,272	807,960
Interest income	(61,747)	(73,723)
Reclassifications from (to) nonaccretable balance, net	10,817	1,000
Other	(9,733)	12,229
Balance at end of period	$ 577,609	$ 747,466