INVESTMENTS IN REAL ESTATE AND OTHER SECURITIES - Summary of Real Estate Securities in an Unrealized Loss Position (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 30,311,608
Weighted Average Life (Years)	2 years 3 months 18 days
Less than 12 Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 10,511,509
Before Impairment - Amortized Cost Basis	3,144,203
Credit Impairment - Amortized Cost Basis	(2,072)
Amortized Cost Basis	3,142,131
Gross Unrealized Losses - Less than 12 Months	(51,194)
Carrying Value - Less than 12 Months	$ 3,090,937
Number of Securities - Less than 12 Months | security	112
Weighted Average Rating	AAA
Weighted Average Coupon	2.30%
Weighted Average Yield	2.05%
Weighted Average Life (Years)	6 years 1 month 6 days
12 or More Months
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 3,758,699
Before Impairment - Amortized Cost Basis	112,034
Credit Impairment - Amortized Cost Basis	(8,297)
Amortized Cost Basis	103,737
Gross Unrealized Losses - 12 or More Months	(20,889)
Carrying Value - 12 or More Months	$ 82,848
Number of Securities - 12 or More Months | security	70
Weighted Average Rating	AA+
Weighted Average Coupon	0.89%
Weighted Average Yield	2.59%
Weighted Average Life (Years)	2 years 4 months 24 days
Total/Weighted Average
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 14,270,208
Before Impairment - Amortized Cost Basis	3,256,237
Credit Impairment - Amortized Cost Basis	(10,369)
Amortized Cost Basis	3,245,868
Gross Unrealized Losses - Total	(72,083)
Carrying Value - Total	$ 3,173,785
Number of Securities - Total | security	182
Weighted Average Rating	AAA
Weighted Average Coupon	2.26%
Weighted Average Yield	2.07%
Weighted Average Life (Years)	6 years