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, 2015	Mar. 31, 2014
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 473,897	$ 575,863
Interest income	(46,483)	(58,961)
Reclassifications from nonaccretable balance, net	293	37
Other	1,610	(838)
Balance at end of period	429,317	516,101
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	76,518	37,230
Interest income	(5,206)	(6,328)
Reclassifications from nonaccretable balance, net	110	37
Balance at end of period	71,422	30,939
Other [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	397,379	538,633
Interest income	(41,277)	(52,633)
Reclassifications from nonaccretable balance, net	183
Other	1,610	(838)
Balance at end of period	$ 357,895	$ 485,162