REAL ESTATE AND OTHER SECURITIES - Summary of Real Estate Securities in an Unrealized Loss Position (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 31,488,837
Weighted Average Life (Years)	1 year 6 months
Less than 12 Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 15,641,371
Before Impairment - Amortized Cost Basis	13,904,964
Credit Impairment - Amortized Cost Basis	(583)
Amortized Cost Basis	13,904,381
Gross Unrealized Losses - Less than 12 Months	(417,210)
Carrying Value - Less than 12 Months	$ 13,487,171
Number of Securities - Less than 12 Months | security	60
Weighted Average Rating	AAA
Weighted Average Coupon	2.20%
Weighted Average Yield	2.20%
Weighted Average Life (Years)	7 years 4 months 24 days
12 or More Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 5,468,704
Before Impairment - Amortized Cost Basis	142,858
Credit Impairment - Amortized Cost Basis	(7,195)
Amortized Cost Basis	135,662
Gross Unrealized Losses - 12 or More Months	(24,604)
Carrying Value - 12 or More Months	$ 111,058
Number of Securities - 12 or More Months | security	88
Weighted Average Rating	AA+
Weighted Average Coupon	1.50%
Weighted Average Yield	1.40%
Weighted Average Life (Years)	3 years
Total/Weighted Average
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 21,110,075
Before Impairment - Amortized Cost Basis	14,047,822
Credit Impairment - Amortized Cost Basis	(7,778)
Amortized Cost Basis	14,040,043
Gross Unrealized Losses - Total	(441,814)
Carrying Value - Total	$ 13,598,229
Number of Securities - Total | security	148
Weighted Average Rating	AAA
Weighted Average Coupon	2.20%
Weighted Average Yield	2.20%
Weighted Average Life (Years)	7 years 4 months 24 days