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LOANS - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	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	$ 76,662,433,000		$ 76,662,433,000		$ 88,455,103,000	[1]
Loan loss (provision) reversal	6,134,000	$ 21,993,000	145,260,000	$ (146,084,000)
Allowance for loan loss	27,700,000		$ 27,700,000		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	1,700,000,000		$ 1,700,000,000		345,800,000
Loans held-for-sale	$ 3,700,000		$ 3,700,000		$ 47,000,000
Percent of adjustable-rate loans	22.00%		22.00%		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		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		4,900,000		(57,400,000)
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		5,400,000	0	(4,500,000)
Loans held-for-sale	$ 2,100,000		2,100,000		$ 0
Unfunded commitments	221,700,000		221,700,000		87,300,000
Unfunded commitments, expected credit loss	500,000		500,000		700,000
Principal balance		9,700,000		29,300,000
Carrying value		9,600,000		21,900,000
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]
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]
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	6,100,000	$ 11,600,000	11,800,000	$ (10,100,000)
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
Residential mortgage loan warehouse facility
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets	$ 1,431,000		$ 1,431,000		$ 0
[1] (1)Derived from the audited consolidated financial statements at December 31, 2020.

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