Investments - Investments Held by Consolidated Funds (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investment [Line Items]
Cost of investments of Consolidated	$ 22,486.0	$ 19,514.9
United States [Member]
Investment [Line Items]
Cost of equity securities	1,901.6	2,160.6
Cost of partnership and LLC interests	2,572.5	2,593.5
Cost of Loans	390.0	361.4
Cost of assets of the CLOs	8,720.7	7,446.8
Canada [Member]
Investment [Line Items]
Cost of equity securities		6.1
Cost of partnership and LLC interests	103.0	112.0
Cost of Loans	0.6	9.5
Cost of assets of the CLOs	206.3	247.1
Europe [Member]
Investment [Line Items]
Cost of equity securities	1,617.8	1,249.3
Cost of partnership and LLC interests	952.0	1,052.6
Cost of Loans		413.3
Cost of assets of the CLOs	5,397.4	3,345.2
Australia [Member]
Investment [Line Items]
Cost of assets of the CLOs	75.7	5.0
Global [Member]
Investment [Line Items]
Cost of equity securities	89.0	85.3
Cost of partnership and LLC interests	412.8	427.2
Cost of assets of the CLOs	$ 46.6