Loans and leases and the allowance for credit losses - Recorded Investment in Loans and Leases on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 65,924,267	$ 66,570,957
Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,090,391	1,065,551
Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	64,408,644	65,058,292
Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	425,232	447,114
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	17,469,138	17,776,953
Commercial, Financial, Leasing, etc. [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	212,054	159,761
Commercial, Financial, Leasing, etc. [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	17,240,343	17,599,755
Commercial, Financial, Leasing, etc. [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	16,741	17,437
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	25,944,819	25,993,790
Commercial Real Estate [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	461,084	480,335
Commercial Real Estate [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	25,117,184	25,123,758
Commercial Real Estate [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	366,551	389,697
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,094,577	11,240,837
Residential Real Estate [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	343,642	349,477
Residential Real Estate [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	10,712,226	10,854,591
Residential Real Estate [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	38,709	36,769
Consumer loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,415,733	11,559,377
Consumer loans [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	73,611	75,978
Consumer loans [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,338,891	11,480,188
Consumer loans [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 3,231	$ 3,211