INVESTMENTS IN REAL ESTATE AND OTHER SECURITIES - Summary of Real Estate Securities in an Unrealized Loss Position (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 27,043,910
Weighted Average Life (Years)	1 year 2 months 12 days
Less than 12 Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 8,860,380
Before Impairment - Amortized Cost Basis	1,005,995
Credit Impairment - Amortized Cost Basis	(8,561)
Amortized Cost Basis	997,434
Gross Unrealized Losses - Less than 12 Months	(98,478)
Carrying Value - Less than 12 Months	$ 898,956
Number of Securities - Less than 12 Months | security	184
Weighted Average Rating	BBB+
Weighted Average Coupon	3.34%
Weighted Average Yield	4.84%
Weighted Average Life (Years)	11 years 6 months
12 or More Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 2,381,211
Before Impairment - Amortized Cost Basis	96,175
Credit Impairment - Amortized Cost Basis	(10,454)
Amortized Cost Basis	85,721
Gross Unrealized Losses - 12 or More Months	(13,057)
Carrying Value - 12 or More Months	$ 72,664
Number of Securities - 12 or More Months | security	60
Weighted Average Rating	A
Weighted Average Coupon	1.80%
Weighted Average Yield	2.87%
Weighted Average Life (Years)	2 years 7 months 6 days
Total/Weighted Average
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 11,241,591
Before Impairment - Amortized Cost Basis	1,102,170
Credit Impairment - Amortized Cost Basis	(19,015)
Amortized Cost Basis	1,083,155
Gross Unrealized Losses - Total	(111,535)
Carrying Value - Total	$ 971,620
Number of Securities - Total | security	244
Weighted Average Rating	BBB+
Weighted Average Coupon	3.22%
Weighted Average Yield	4.69%
Weighted Average Life (Years)	10 years 9 months 18 days