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LOANS - Additional Information (Details)	12 Months Ended
LOANS - Additional Information (Details)	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2020 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	$ 88,455,103,000	$ 130,295,081,000
Loan loss provision	147,581,000	16,569,000	$ 3,496,000
Allowance for loan loss	169,500,000	$ 20,100,000
Loans	$ 498,081,000
Loans receivable with variable rates of interest	94.00%	92.00%
Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss		$ 37,400,000
Residential Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	$ 300,000,000	1,600,000,000
Loans	$ 47,000,000.0	$ 66,700,000
Percent of adjustable-rate loans	37.00%	36.00%
Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision	$ 78,400,000	$ 9,200,000
Aggregate principal balance	181,200,000	43,600,000
Carrying value	$ 113,600,000	30,900,000
Number of contracts modified \| loan	5
Number of contracts modified, partial paydowns \| loan	2
Restructuring, partial paydowns	$ 4,500,000
Future funding commitments	4,100,000
Amortized cost basis, nonaccrual	46,800,000	175,200,000
Interest income, nonaccrual	2,100,000	(100,000)
Unfunded corporate loan commitments	99,300,000	181,400,000
Expected gain (loss) on unfunded commitments	5,100,000
Restructuring, carrying value	243,800,000
Commercial Mortgage Loans \| Extended Maturity
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss	$ 23,600,000
Number of contracts modified \| loan	4
Commercial Mortgage Loans \| Payment Deferral
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of contracts modified \| loan	1
Forbearance term	120 days
Commercial Mortgage Loans \| Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss				$ 7,800,000
Commercial Mortgage Loans \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision	$ 54,800,000
Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision	4,500,000	7,400,000	$ 0
Aggregate principal balance		19,600,000
Carrying value		12,200,000
Loans	2,239,930,000	2,144,850,000
Amortized cost basis, nonaccrual	0.0	12,200,000
Interest income, nonaccrual	0.0	1,500,000
Unfunded corporate loan commitments	87,300,000	81,200,000
Debt instrument face amount	4,800,000
Write offs	11,900,000
Expected gain (loss) on unfunded commitments	700,000
Corporate Loans \| Second lien loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	750,805,000	748,710,000
Debt instrument face amount	2,800,000
Corporate Loans \| First lien loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	1,489,125,000	$ 1,396,140,000
Write offs	19,600,000
Corporate Loans \| Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss				$ 29,700,000
Corporate Loans \| Cumulative Effect, Period of Adoption, Adjusted Balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision	$ 9,900,000
Minimum \| Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity	5 years
Maximum \| Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity	7 years