INVESTMENTS - Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Other investments, at fair value	$ 20,949	$ 16,320	$ (8,487)
Total investment income	146,569	157,110	201,289
Funds
Schedule of Equity Method Investments [Line Items]
Equity-method investments	68,145	66,922	138,465
Companies
Schedule of Equity Method Investments [Line Items]
Equity-method investments	$ 57,475	$ 73,868	$ 71,311