Allowance for credit losses - Recorded Investment in Loans and Leases on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 66,570,957	$ 60,096,005
Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,065,551	1,110,873
Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	65,058,292	58,331,770
Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	447,114	653,362
Commercial, Financial, Leasing, etc [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	17,776,953	15,734,436
Commercial, Financial, Leasing, etc [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	159,761	171,442
Commercial, Financial, Leasing, etc [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	17,599,755	15,539,232
Commercial, Financial, Leasing, etc [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	17,437	23,762
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	25,993,790	24,411,114
Commercial Real Estate [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	480,335	561,615
Commercial Real Estate [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	25,123,758	23,281,585
Commercial Real Estate [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	389,697	567,914
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,240,837	7,923,165
Residential Real Estate [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	349,477	306,320
Residential Real Estate [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	10,854,591	7,560,104
Residential Real Estate [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	36,769	56,741
Consumer loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,559,377	12,027,290
Consumer loans [Member] | Individually evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	75,978	71,496
Consumer loans [Member] | Collectively evaluated for impairment [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,480,188	11,950,849
Consumer loans [Member] | Purchased impaired [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 3,211	$ 4,945