Pledged assets - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 27,200.0		$ 27,900.0
Fair value of accepted collateral	280.9		74.7
Amount of securities repledged or sold collateral	0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,100.0	331.5
US Government Corporations and Agencies Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	23,900.0		21,500.0
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	412.8		460.2
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	2,900.0		6,000.0
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	4,200.0		4,800.0
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 249.6		$ 1,200.0