Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended					9 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Standby Letter of Credit	Dec. 31, 2010 Standby Letter of Credit	Dec. 31, 2011 Venture capital and private equity fund investments	Dec. 31, 2011 Venture capital and private equity fund investments Lower Limit	Dec. 31, 2011 Venture capital and private equity fund investments Upper Limit
Commitments and Contingencies Disclosure [Line Items]
Rent expense for premises and equipment leased under operating leases	$ 11.9	$ 11.5	$ 10.7
Deferred fees				6.1	5.2
Collateral in the form of cash				271.7
Collateral in the form of investment securities				$ 15.4
Commitments to invest, period from the inception of the fund						10 years
Expected commitment period to invest in venture capital and private equity funds (in years)							5 years	7 years