Investments - Schedule of Investments as a Percentage of Investments of Consolidated Funds (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 17,064.7	$ 16,052.2
Percentage of investments in Hedge Funds	100.00%	100.00%
Total assets of the CLOs	$ 5,000.0
Investments	24,018.5	$ 26,044.7
Investments of Consolidated Funds	23,998.8	26,028.8
Loans | Retailing
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 6.8	$ 109.1
Percentage of loans payable of Consolidated Funds	0.03%	0.42%
United States
Schedule of Investments [Line Items]
Percentage of investments in Hedge Funds	69.28%	68.53%
Investments	$ 16,628.6	$ 17,838.9
United States | Equity securities
Schedule of Investments [Line Items]
Equity securities	$ 590.8	$ 1,333.8
Percentage of equity securities	2.45%	5.12%
United States | Equity securities | Commercial & Professional Services
Schedule of Investments [Line Items]
Equity securities	$ 0.2	$ 201.3
Percentage of equity securities	0.00%	0.77%
United States | Equity securities | Diversified Financials
Schedule of Investments [Line Items]
Equity securities	$ 290.2	$ 264.4
Percentage of equity securities	1.21%	1.02%
United States | Equity securities | Food, Beverage & Tobacco
Schedule of Investments [Line Items]
Equity securities	$ 0.0	$ 414.0
Percentage of equity securities	0.00%	1.59%
United States | Equity securities | Media
Schedule of Investments [Line Items]
Equity securities	$ 69.3	$ 97.3
Percentage of equity securities	0.29%	0.37%
United States | Equity securities | Health Care Equipment & Services
Schedule of Investments [Line Items]
Equity securities	$ 71.7	$ 100.9
Percentage of equity securities	0.30%	0.39%
United States | Equity securities | Consumer Services
Schedule of Investments [Line Items]
Equity securities	$ 38.6	$ 67.7
Percentage of equity securities	0.16%	0.26%
United States | Equity securities | Capital Goods
Schedule of Investments [Line Items]
Equity securities	$ 0.6	$ 60.3
Percentage of equity securities	0.00%	0.23%
United States | Equity securities | Software & Services
Schedule of Investments [Line Items]
Equity securities	$ 65.4	$ 38.9
Percentage of equity securities	0.27%	0.15%
United States | Equity securities | Transportation
Schedule of Investments [Line Items]
Equity securities	$ 3.1	$ 49.6
Percentage of equity securities	0.01%	0.19%
United States | Equity securities | Food & Staples Retailing
Schedule of Investments [Line Items]
Equity securities	$ 0.0	$ 30.9
Percentage of equity securities	0.00%	0.12%
United States | Equity securities | Consumer Durables & Apparel
Schedule of Investments [Line Items]
Equity securities	$ 50.6	$ 7.6
Percentage of equity securities	0.21%	0.03%
United States | Equity securities | Other
Schedule of Investments [Line Items]
Equity securities	$ 1.1	$ 0.9
Percentage of equity securities	0.00%	0.00%
United States | Loans
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 13.4	$ 211.5
Percentage of loans payable of Consolidated Funds	0.06%	0.82%
United States | Loans | Commercial & Professional Services
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 0.1	$ 31.1
Percentage of loans payable of Consolidated Funds	0.00%	0.12%
United States | Loans | Diversified Financials
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 0.0	$ 6.7
Percentage of loans payable of Consolidated Funds	0.00%	0.03%
United States | Loans | Transportation
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 0.0	$ 27.8
Percentage of loans payable of Consolidated Funds	0.00%	0.11%
United States | Loans | Materials
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 0.6	$ 32.5
Percentage of loans payable of Consolidated Funds	0.00%	0.12%
United States | Loans | Other
Schedule of Investments [Line Items]
Loans payable of Consolidated Funds	$ 5.9	$ 4.3
Percentage of loans payable of Consolidated Funds	0.03%	0.02%
United States | Partnership and LLC interests
Schedule of Investments [Line Items]
Partnership and LLC interests	$ 1,889.3	$ 2,188.5
Percentage of partnership and LLC interests	7.87%	8.41%
United States | Total investment in Hedge Funds
Schedule of Investments [Line Items]
Investment in Hedge Funds	$ 2,841.2	$ 3,753.5
Percentage of investments in Hedge Funds	11.84%	14.42%
United States | Assets of the CLOs - Bonds
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 50.6	$ 141.8
Percentage of assets of the CLOs	0.21%	0.54%
United States | Assets of the CLOs - Equity
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 2.3	$ 6.5
Percentage of assets of the CLOs	0.01%	0.02%
United States | Assets of the CLOs - Loans
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 11,241.0	$ 10,203.3
Percentage of assets of the CLOs	46.84%	39.20%
United States | Total assets of the CLOs
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 11,293.9	$ 10,351.6
Percentage of assets of the CLOs	47.06%	39.76%
Europe
Schedule of Investments [Line Items]
Percentage of investments in Hedge Funds	25.09%	27.33%
Investments of Consolidated Funds	$ 6,021.2	$ 7,112.3
Europe | Equity securities
Schedule of Investments [Line Items]
Equity securities	$ 418.6	$ 1,011.3
Percentage of equity securities	1.74%	3.89%
Europe | Equity securities | Commercial & Professional Services
Schedule of Investments [Line Items]
Equity securities	$ 43.0	$ 75.4
Percentage of equity securities	0.18%	0.29%
Europe | Equity securities | Media
Schedule of Investments [Line Items]
Equity securities	$ 10.5	$ 40.0
Percentage of equity securities	0.04%	0.15%
Europe | Equity securities | Health Care Equipment & Services
Schedule of Investments [Line Items]
Equity securities	$ 81.0	$ 97.8
Percentage of equity securities	0.34%	0.38%
Europe | Equity securities | Consumer Services
Schedule of Investments [Line Items]
Equity securities	$ 94.7	$ 38.2
Percentage of equity securities	0.39%	0.15%
Europe | Equity securities | Retailing
Schedule of Investments [Line Items]
Equity securities	$ 95.7	$ 119.4
Percentage of equity securities	0.40%	0.46%
Europe | Equity securities | Transportation
Schedule of Investments [Line Items]
Equity securities	$ 0.0	$ 88.8
Percentage of equity securities	0.00%	0.34%
Europe | Equity securities | Food & Staples Retailing
Schedule of Investments [Line Items]
Equity securities	$ 57.9	$ 350.4
Percentage of equity securities	0.24%	1.35%
Europe | Equity securities | Energy
Schedule of Investments [Line Items]
Equity securities	$ 0.0	$ 168.8
Percentage of equity securities	0.00%	0.65%
Europe | Equity securities | Other
Schedule of Investments [Line Items]
Equity securities	$ 35.8	$ 32.5
Percentage of equity securities	0.15%	0.12%
Europe | Partnership and LLC interests
Schedule of Investments [Line Items]
Partnership and LLC interests	$ 640.6	$ 800.0
Percentage of investments in Hedge Funds	2.67%	3.07%
Europe | Assets of the CLOs - Bonds
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 1,115.8	$ 1,081.3
Percentage of assets of the CLOs	4.65%	4.15%
Europe | Assets of the CLOs - Equity
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 3.8	$ 9.7
Percentage of assets of the CLOs	0.02%	0.04%
Europe | Assets of the CLOs - Loans
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 3,837.4	$ 4,208.5
Percentage of assets of the CLOs	15.99%	16.17%
Europe | Assets of the CLOs - Other
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 5.0	$ 1.5
Percentage of assets of the CLOs	0.02%	0.01%
Europe | Total assets of the CLOs
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 4,962.0	$ 5,301.0
Percentage of assets of the CLOs	20.68%	20.37%
Global
Schedule of Investments [Line Items]
Percentage of investments in Hedge Funds	5.63%	4.14%
Investments of Consolidated Funds	$ 1,349.0	$ 1,077.6
Global | Equity securities | Food, Beverage & Tobacco
Schedule of Investments [Line Items]
Equity securities	$ 126.2	$ 110.8
Percentage of equity securities	0.53%	0.43%
Global | Partnership and LLC interests
Schedule of Investments [Line Items]
Partnership and LLC interests	$ 613.4	$ 492.5
Percentage of partnership and LLC interests	2.56%	1.89%
Global | Assets of the CLOs - Bonds
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 14.5	$ 12.7
Percentage of assets of the CLOs	0.06%	0.05%
Global | Assets of the CLOs - Loans
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 594.9	$ 461.6
Percentage of assets of the CLOs	2.48%	1.77%
Global | Total assets of the CLOs
Schedule of Investments [Line Items]
Total assets of the CLOs	$ 609.4	$ 474.3
Percentage of assets of the CLOs	2.54%	1.82%