Loans and leases - Summary of Changes in Accretable Yield for Acquired Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Purchased Impaired [Member]
Credit Derivatives [Line Items]
Balance at beginning of period	$ 30,805	$ 9,245	$ 4,318
Additions		39,358	4,922
Interest income	(40,551)	(26,221)	(5,826)
Reclassifications from (to) nonaccretable balance, net	51,998	8,629	5,831
Other		(206)
Balance at end of period	42,252	30,805	9,245
Other [Member]
Credit Derivatives [Line Items]
Balance at beginning of period	807,960	447,505	531,546
Additions		648,631	13,747
Interest income	(295,654)	(268,315)	(171,207)
Reclassifications from (to) nonaccretable balance, net	148,490	1,800	34,000
Other	(22,524)	(21,661)	39,419
Balance at end of period	$ 638,272	$ 807,960	$ 447,505