Accrued Performance Fees - Components of Accrued Giveback Obligations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Operating Statistics [Line Items]
Accrued giveback obligations	$ (69.2)	$ (136.5)
Corporate Private Equity [Member]
Operating Statistics [Line Items]
Accrued giveback obligations	(18.9)	(77.8)
Global Market Strategies [Member]
Operating Statistics [Line Items]
Accrued giveback obligations	(2.1)	(1.2)
Real Assets [Member]
Operating Statistics [Line Items]
Accrued giveback obligations	$ (48.2)	$ (57.5)