Loans and leases and the allowance for credit losses - Outstanding Principal Balance and Carrying Amount of Loans that is Included in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Outstanding principal and carrying value of acquired loans recorded at fair value
Outstanding principal balance	$ 6,148,875	$ 6,705,120
Carrying amount	5,315,134	5,840,353
Commercial, Financial, Leasing, etc. [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	754,913	928,107
Real Estate Commercial [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	2,322,317	2,567,050
Residential Real Estate [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	677,306	707,309
Consumer [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	$ 1,560,598	$ 1,637,887