INVESTMENTS - Fair Value, Additional Information (Details) (Phantom) - Consolidated Funds - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Investments In Marketable Securities [Line Items]
Securities sold short, proceeds	$ 0	$ 2,644
United States
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	4,046,820	4,019,823
Total equity securities, cost	8,862	6,117
Real estate securities, cost	57,080	0
Europe
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	2,642,902	2,477,821
Total equity securities, cost	40,078	320
Asia and other
Schedule Of Investments In Marketable Securities [Line Items]
Total debt securities, cost	184,948	233,603
Total equity securities, cost	$ 3,880	$ 22,977