Investments - Net Investment Gains Losses of Consolidated Funds (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Losses from liabilities of CLOs	$ (926.2)	$ (56.7)	$ (752.4)
Total	1,758.0	(323.3)	(245.4)
Collateralized Loan Obligations [Member]
Schedule of Investments [Line Items]
Gains (losses) from investments of Consolidated Funds	2,680.6	(260.8)	502.0
Losses from liabilities of CLOs	(927.8)	(64.2)	(752.4)
Gains on other assets of CLOs	5.2	1.7	5.0
Total	$ 1,758.0	$ (323.3)	$ (245.4)