Loans and leases and the allowance for credit losses - Allocation of Allowance for Credit Losses on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 927,117	$ 925,860
Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	852,782	852,085
Unallocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	74,335	73,775
Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	133,974	131,656
Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	715,691	717,064
Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	3,117	3,365
Commercial, Financial, Leasing, etc. [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	257,851	246,759
Commercial, Financial, Leasing, etc. [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	45,277	33,669
Commercial, Financial, Leasing, etc. [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	212,574	212,930
Commercial, Financial, Leasing, etc. [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses		160
Commercial Real Estate [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	328,016	337,101
Commercial Real Estate [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	45,503	55,291
Commercial Real Estate [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	281,492	280,789
Commercial Real Estate [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	1,021	1,021
Residential Real Estate [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	90,122	88,807
Residential Real Estate [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	22,444	20,502
Residential Real Estate [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	66,145	66,684
Residential Real Estate [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	1,533	1,621
Consumer loans [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	176,793	179,418
Consumer loans [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	20,750	22,194
Consumer loans [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	155,480	156,661
Consumer loans [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 563	$ 563