Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Venture capital and private equity fund investments	Sep. 30, 2012 Venture capital and private equity fund investments Lower Limit	Sep. 30, 2012 Venture capital and private equity fund investments Upper Limit	Sep. 30, 2012 Standby Letter of Credit	Dec. 31, 2011 Standby Letter of Credit
Commitments and Contingencies Disclosure [Line Items]
Deferred fees				$ 5.0	$ 6.1
Collateral in the form of cash				347.7
Collateral in the form of investment securities				$ 13.2
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