INVESTMENTS - Fair Value, Additional Information (Details) - Consolidated Funds - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Investments In Marketable Securities [Line Items]
Securities sold short, proceeds	$ 71,554	$ 41,541
United States
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	3,214,637	2,378,759
Total equity securities, cost	9,327	5,462
Europe
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	1,663,145	1,214,068
Total equity securities, cost	9,278	1,494
Asia and other
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	59,156	57,400
Total equity securities, cost	$ 203,465	$ 118,292