INVESTMENTS - Investment income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule of Equity Method Investments [Line Items]
Other investments, at fair value	$ (4,862)	$ 13,695	$ 857	$ 2,854
Total investment income	32,835	56,923	94,985	91,486
Funds
Schedule of Equity Method Investments [Line Items]
Equity-method investments	17,997	25,798	57,317	53,064
Companies
Schedule of Equity Method Investments [Line Items]
Equity-method investments	$ 19,700	$ 17,430	$ 36,811	$ 35,568