INVESTMENTS - Investment income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Other investments, at fair value	$ (2,916)	$ 18,817	$ (11,393)	$ 19,674
Total investment income (loss)	60,296	26,819	18,030	121,804
Funds
Schedule of Equity Method Investments [Line Items]
Equity-method investments:	63,225	(12,106)	29,114	45,211
Companies
Schedule of Equity Method Investments [Line Items]
Equity-method investments:	$ (13)	$ 20,108	$ 309	$ 56,919