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Pledged assets - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged $ 27,400.0   $ 27,400.0   $ 27,900.0
Fair value of accepted collateral 537.4   537.4   74.7
Amount of securities repledged or sold collateral 0   0   0
Average deposits maintained to meet Federal Reserve Bank reserve requirements 1,100.0 467.5 1,100.0 307.5  
US Government Corporations and Agencies Securities
         
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged 21,600.0   21,600.0   21,500.0
Obligations of States and Political Subdivisions
         
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged 351.8   351.8   460.2
Loans
         
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged 5,400.0   5,400.0   6,000.0
Collateral Requirements
         
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged 4,000.0   4,000.0   4,800.0
Available for Sale Investment Securities
         
Financial Instruments Owned and Pledged as Collateral [Line Items]          
Amount of securities and loans pledged $ 367.0   $ 367.0   $ 1,200.0