INVESTMENTS - Investment income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Other investments, at fair value	$ (4,167)	$ 5,719
Total investment (loss) income	(109,673)	62,150
Funds
Schedule of Equity Method Investments [Line Items]
Equity-method investments:	(105,634)	39,320
Companies
Schedule of Equity Method Investments [Line Items]
Equity-method investments:	$ 128	$ 17,111