LOANS HELD-FOR-INVESTMENT (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) loan	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) loan	Jan. 01, 2020 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 607,601,000		$ 301,630,000
Number of loans | loan	124
Loans held-for-investment and related receivables, net	$ 627,380,000		301,630,000
Cash and cash equivalents	245,076,000	$ 26,130,000	473,355,000		$ 19,674,000
Transition adjustment on January 1, 2020	19,779,000		0
Provision for credit losses	$ 17,777,000	$ 0
Broadly syndicated loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans purchased | loan	112
Net book value of loans purchased	$ 330,700,000
Number of loans | loan	113
Loans held-for-investment and related receivables, net	$ 333,449,000		2,750,000
Weighted-average maximum years to maturity	5 years 2 months 12 days
Transition adjustment on January 1, 2020	$ 4,429,000		0
Provision for credit losses	4,389,000
Mezzanine loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	140,100,000
Loans held-for-investment and related receivables, net			$ 146,060,000
Number of loans | loan			8
Transition adjustment on January 1, 2020	14,541,000		$ 0
Provision for credit losses	$ 13,047,000
Credit Concentration Risk | Asset Benchmark | Broadly syndicated loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of mezzanine loans to total assets	4.50%
Credit Concentration Risk | Asset Benchmark | Mezzanine loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of mezzanine loans to total assets			3.90%
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other commitment	$ 15,400,000
Reserves For Settlement Of Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 55,600,000
Cumulative Effect, Period Of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transition adjustment on January 1, 2020				$ 2,002,000
Cumulative Effect, Period Of Adoption, Adjustment | Broadly syndicated loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transition adjustment on January 1, 2020				40,000
Cumulative Effect, Period Of Adoption, Adjustment | Mezzanine loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transition adjustment on January 1, 2020				$ 1,494,000