LOANS - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	$ 89,192,411,000		$ 89,192,411,000			$ 130,295,081,000	[1]
Loan loss provision recorded during period	(21,993,000)	$ 3,504,000	146,084,000	$ 9,207,000
Allowance for loan loss	168,000,000.0		168,000,000.0			20,100,000
Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision recorded during period		3,500,000	74,100,000	9,200,000
Principal balance		7,000,000.0	160,200,000	43,600,000
Carrying value		$ 0.0	97,000,000.0	30,900,000
Loans on nonaccrual status	50,600,000		50,600,000			175,200,000
Unfunded commitments	123,200,000		123,200,000			$ 181,400,000
Unfunded commitments, expected credit loss	$ 6,200,000		$ 6,200,000
Loans receivable with variable interest rates	94.00%		94.00%			92.00%
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	$ 9,900,000		$ 9,900,000
Restructuring, carrying value	$ 109,500,000		109,500,000
Number of loans restructured | loan	2
Commercial Mortgage Loans | Payment Deferral, Forbearance
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans restructured | loan	1
Forbearance duration	120 days
Commercial Mortgage Loans | Impact of adopting CECL
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 | CECL provision, adjusted balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision recorded during period	$ (4,900,000)		57,400,000
Residential Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	200,000		200,000			$ 1,600,000,000
Loans	$ 50,000,000.0		$ 50,000,000.0			$ 66,700,000
Percent of adjustable-rate loans	35.00%		35.00%			36.00%
Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision recorded during period	$ (5,400,000)		$ 4,500,000	$ 0
Loans	2,061,878,000		2,061,878,000			$ 2,144,850,000		$ 1,887,182,000
Principal balance	9,700,000		29,300,000
Carrying value	9,600,000		21,900,000
Loans on nonaccrual status	12.9		12.9			12,200,000
Debt	4,800,000		4,800,000
Allowance for loan losses, writeoffs			11,900,000
Unfunded commitments	61,300,000		61,300,000			81,200,000
Unfunded commitments, expected credit loss	700,000		700,000
Corporate Loans | Impact of adopting CECL
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 | CECL provision, adjusted balance
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision recorded during period	(11,600,000)		$ 10,100,000
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity (in years)			5 years
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity (in years)			7 years
First lien loans | Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	1,320,115,000		$ 1,320,115,000			1,396,140,000		1,346,356,000
Allowance for loan losses, writeoffs			19,600,000
Second lien loans | Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	741,763,000		741,763,000			$ 748,710,000		$ 540,826,000
Debt	$ 2,800,000		$ 2,800,000

[1] Derived from the audited consolidated financial statements at December 31, 2019.