Loans and leases and the allowance for credit losses - Loans and Leases Receivable Allowance by Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 921,223	$ 908,290
Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	847,604	836,695
Unallocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	73,619	71,595
Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	135,938	165,579
Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	706,306	663,674
Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	5,360	7,442
Commercial, financial, leasing, etc [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	237,467	234,022
Commercial, financial, leasing, etc [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	29,738	48,517
Commercial, financial, leasing, etc [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	207,556	185,048
Commercial, financial, leasing, etc [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	173	457
Commercial real estate loans [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	344,851	367,637
Commercial real estate loans [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	59,794	71,784
Commercial real estate loans [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	283,547	291,271
Commercial real estate loans [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	1,510	4,582
Residential real estate [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	93,968	91,915
Residential real estate [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	23,953	29,420
Residential real estate [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	66,724	60,742
Residential real estate [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	3,291	1,753
Consumer loans [Member] | Allocated [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	171,318	143,121
Consumer loans [Member] | Individually evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	22,453	15,858
Consumer loans [Member] | Collectively evaluated for impairment [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	148,479	126,613
Consumer loans [Member] | Purchased impaired [Member]
Loans And Leases Receivable [Line Items]
Allowance for credit losses	$ 386	$ 650