GOODWILL AND INTANGIBLE ASSETS (Narrative) (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) contract	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Feb. 21, 2020 contract	Dec. 31, 2019 USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 250,467,000			$ 250,467,000			$ 19,217,000
Number of collateral management contracts acquired | contract		7				7
Fully-amortized intangibles, amount removed during the period		$ 4,700,000
Acquisitions				224,601,000
Goodwill impairment				0	$ 0
Strategic Initiatives
Acquired Finite-Lived Intangible Assets [Line Items]
Acquisitions				224,601,000
General, administrative and other expense
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization expense	11,100,000		$ 600,000	13,700,000	$ 3,000,000.0
Crestline Denali
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated useful lives, intangible assets		5 years 10 months 24 days
Finite-lived intangible assets acquired		$ 34,700,000
Energy Investors Funds
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment charge	20,000,000.0
Management contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	214,247,000			$ 214,247,000			12,498,000
Estimated useful lives, intangible assets				5 years 8 months 12 days
Management contracts | SSG Capital Holdings
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 171,700,000			$ 171,700,000
Estimated useful lives, intangible assets	5 years 7 months 6 days
Client relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 25,141,000			$ 25,141,000			6,341,000
Estimated useful lives, intangible assets				9 years 3 months 18 days
Client relationships | SSG Capital Holdings
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 18,800,000			$ 18,800,000
Estimated useful lives, intangible assets	9 years 9 months 18 days
Trade name
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 11,079,000			$ 11,079,000			$ 378,000
Estimated useful lives, intangible assets				9 years 7 months 6 days
Trade name | SSG Capital Holdings
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 10,700,000			$ 10,700,000
Estimated useful lives, intangible assets	9 years 9 months 18 days
Client Relationships and Trade Names | Energy Investors Funds
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets removed	$ 35,100,000