INVESTMENTS - Investment income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Other investments, at fair value	$ (1,113)	$ 668	$ (5,762)	$ 10,107
Total investment income	51,061	65,758	150,618	136,205
Funds
Schedule of Equity Method Investments [Line Items]
Equity-method investments	34,733	47,323	104,216	76,542
Companies
Schedule of Equity Method Investments [Line Items]
Equity-method investments	$ 17,441	$ 17,767	$ 52,164	$ 49,556