SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Impact of Incentive Fees) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments			$ 1,724,571
Other assets			130,341
Total assets	$ 8,560,082		8,563,522
Total liabilities	6,994,887		7,103,230
Cumulative effect of the adoption of ASC 606			0
Total equity	1,565,195		1,460,292
Total liabilities and equity	8,560,082		8,563,522
ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of the adoption of ASC 606			$ (22,611)
Adjustments | ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments		$ 0
Other assets	23,704	(22,611)
Total assets	23,454	(22,611)
Total liabilities		0
Cumulative effect of the adoption of ASC 606		(22,611)
Total equity	23,454	(22,611)
Total liabilities and equity	23,454	(22,611)
Balances without adoption of ASC 606 | ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments		1,724,571
Other assets		107,730
Total assets	8,583,536	8,540,911
Total liabilities		7,103,230
Cumulative effect of the adoption of ASC 606		(22,611)
Total equity	1,588,649	1,437,681
Total liabilities and equity	$ 8,583,536	$ 8,540,911