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	Sep. 30, 2015	Dec. 31, 2014
Outstanding principal and carrying value of acquired loans recorded at fair value
Outstanding principal balance	$ 2,410,454	$ 3,070,268
Carrying amount	2,029,961	2,596,545
Commercial, Financial, Leasing, etc. [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	103,583	247,820
Commercial Real Estate [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	728,376	961,828
Residential Real Estate [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	385,885	453,360
Consumer [Member]
Outstanding principal and carrying value of acquired loans recorded at fair value
Carrying amount	$ 812,117	$ 933,537