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LOANS - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) loan		Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	$ 82,376,305,000		$ 82,376,305,000		$ 88,455,103,000	[1]
Loan loss provision (reversal)	494,000	$ 68,751,000	(139,126,000)	$ 168,077,000
Allowance for loan loss	33,900,000		$ 33,900,000		169,500,000
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)			8 years
Cumulative effect of change in accounting principle for credit losses
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	$ 1,000,000,000.0		$ 1,000,000,000.0		$ 345,800,000
Percent of adjustable-rate loans	31.00%		31.00%		37.00%
Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision (reversal)		22,000,000.0		74,100,000
Current period (allowance) reversal			$ 67,400,000
Number of loans restructured | loan					5
Restructuring, carrying value					$ 243,800,000
Number of contracts modified, partial paydowns | loan					2
Restructuring, partial paydowns					$ 4,500,000
Future funding commitments					4,100,000
Loans on nonaccrual status					46,800,000
Interest income, nonaccrual					2,100,000
Unfunded commitments					99,300,000
Unfunded commitments, expected credit loss					$ 5,100,000
Loans receivable with variable interest rates					94.00%
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					$ 23,600,000
Number of loans restructured | loan					4
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 | 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 (reversal)		39,100,000		62,300,000
Current period (allowance) reversal			62,500,000
Commercial Mortgage Loans | Cumulative effect of change in accounting principle for credit losses
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss							$ 7,800,000
Corporate Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan loss provision (reversal)			0	10,000,000.0
Loans	$ 2,066,709,000		2,066,709,000		$ 2,239,930,000
Unfunded commitments	228,200,000		228,200,000		87,300,000
Unfunded commitments, expected credit loss	1,200,000		1,200,000		700,000
Principal balance				29,300,000
Carrying value				4,300,000
Debt		4,800,000		4,800,000
Allowance for loan losses, writeoffs				11,900,000
Corporate Loans | Second lien loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	670,714,000		670,714,000		750,805,000
Debt		2,800,000		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	1,395,995,000		1,395,995,000		$ 1,489,125,000
Allowance for loan losses, writeoffs				19,600,000
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 (reversal)	$ 500,000	$ 7,600,000	$ (5,700,000)	$ 21,700,000
Corporate Loans | Cumulative effect of change in accounting principle for credit losses
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Allowance for loan loss							$ 29,700,000
[1] Derived from the audited consolidated financial statements at December 31, 2020.

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