Loans and leases and the allowance for credit losses - Outstanding Principal Balance and Carrying Amount of Loans that is Included in Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Outstanding principal balance	$ 7,338,476	$ 9,203,366
Carrying Amount	6,383,467	8,159,787
Commercial, financial, leasing, etc [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Carrying Amount	977,430	1,331,198
Real Estate Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Carrying Amount	2,962,899	3,879,518
Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Carrying Amount	742,977	915,371
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Carrying Amount	$ 1,700,161	$ 2,033,700