Investments - Equity Method Investments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Equity method investments	$ 855.1	$ 419.9
Corporate Private Equity [Member]
Schedule of Investments [Line Items]
Equity method investments	251.6	238.5
Global Market Strategies [Member]
Schedule of Investments [Line Items]
Equity method investments	18.0	11.9
Real Assets [Member]
Schedule of Investments [Line Items]
Equity method investments	$ 585.5	$ 169.5