Loans and leases - Outstanding Principal Balance and Carrying Amount of Loans that is Included in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Outstanding principal and carrying value of acquired loans recorded at fair value
Outstanding principal balance	$ 6,705,120	$ 9,203,366
Carrying amount	5,840,353	8,159,787
Commercial, Financial, Leasing, etc [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	928,107	1,331,198
Real Estate Commercial [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	2,567,050	3,879,518
Residential Real Estate [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	707,309	915,371
Consumer [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	$ 1,637,887	$ 2,033,700