Allowance for credit losses - Allocation of Allowance for Credit Losses on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 925,860	$ 908,290
Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	131,656	165,579
Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	717,064	663,674
Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	3,365	7,442
Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	852,085	836,695
Unallocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	73,775	71,595
Commercial, Financial, Leasing, etc [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	33,669	48,517
Commercial, Financial, Leasing, etc [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	212,930	185,048
Commercial, Financial, Leasing, etc [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	160	457
Commercial, Financial, Leasing, etc [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	246,759	234,022
Commercial Real Estate [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	55,291	71,784
Commercial Real Estate [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	280,789	291,271
Commercial Real Estate [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	1,021	4,582
Commercial Real Estate [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	337,101	367,637
Residential Real Estate [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	20,502	29,420
Residential Real Estate [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	66,684	60,742
Residential Real Estate [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	1,621	1,753
Residential Real Estate [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	88,807	91,915
Consumer loans [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	22,194	15,858
Consumer loans [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	156,661	126,613
Consumer loans [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	563	650
Consumer loans [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 179,418	$ 143,121