Investments - Schedule of Income (Loss) from Equity-Method Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investments [Line Items]
Income (loss) from equity-method investments	$ (14.7)	$ 4.3	$ 0.5	$ 32.3
Operating Segments | Corporate Private Equity
Schedule of Investments [Line Items]
Income (loss) from equity-method investments	(10.0)	12.1	20.2	40.1
Operating Segments | Global Market Strategies
Schedule of Investments [Line Items]
Income (loss) from equity-method investments	(1.3)	0.7	(0.4)	2.4
Operating Segments | Real Assets
Schedule of Investments [Line Items]
Income (loss) from equity-method investments	$ (3.4)	$ (8.5)	$ (19.3)	$ (10.2)