INVESTMENT IN SELF-STORAGE REAL ESTATE VENTURE (Equity Method Investments) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Jun. 24, 2020	Jun. 23, 2020	Jan. 28, 2019	Jan. 27, 2019	Mar. 31, 2016
Real Estate Properties [Line Items]
Investment commitment	$ 314,461	$ 608,851
Funded Investment	283,292	478,634
Fair value of investments	316,254	549,684
Funded Investment [Member]
Real Estate Properties [Line Items]
Funded Investment	283,292	478,634
Unfunded Commitment [Member]
Real Estate Properties [Line Items]
Investment commitment	52,450	136,104
SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment commitment	12,300	12,300
SL1 Venture [Member] | Unfunded Commitment [Member]
Real Estate Properties [Line Items]
Investment commitment	$ 40	200
SL1 Venture [Member] | Loan Investments [Member] | Washington DC [Member]
Real Estate Properties [Line Items]
Equity method investment, ownership percentage	49.90%
SL1 Venture [Member] | Loan Investments [Member] | Columbia [Member]
Real Estate Properties [Line Items]
Equity method investment, ownership percentage			100.00%	49.90%
Development Property Investments [Member] | Loan Investments [Member]
Real Estate Properties [Line Items]
Investment commitment	$ 259,490	553,880
Fair value of investments	$ 253,432	$ 490,651
Percentage of completion of construction	40.00%	40.00%
Equity method investment, ownership percentage	49.90%
Development Property Investments [Member] | Loan Investments [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Investment Closing Date	9/14/2017	9/14/2017
Investment commitment	$ 14,657	$ 14,657
Fair value of investments	$ 15,657	$ 13,373
Development Property Investments [Member] | Loan Investments [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Investment Closing Date	10/12/2017	10/12/2017
Investment commitment	$ 0	$ 9,459
Fair value of investments	1,459	$ 1,280
Development Property Investments [Member] | Loan Investments [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Investment Closing Date		3/1/2017
Investment commitment		$ 9,952
Fair value of investments		13,635
Development Property Investments [Member] | Loan Investments [Member] | Funded Investment [Member]
Real Estate Properties [Line Items]
Funded Investment	223,774	422,034
Development Property Investments [Member] | Loan Investments [Member] | Funded Investment [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Funded Investment	14,463	12,618
Development Property Investments [Member] | Loan Investments [Member] | Funded Investment [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Funded Investment	1,661	1,494
Development Property Investments [Member] | Loan Investments [Member] | Funded Investment [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Funded Investment		9,383
Development Property Investments [Member] | Loan Investments [Member] | Unfunded Commitment [Member]
Real Estate Properties [Line Items]
Investment commitment	51,801	135,455
Development Property Investments [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Investment commitment	194	2,039
Development Property Investments [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Investment commitment	0	8,045
Development Property Investments [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Investment commitment		569
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member]
Real Estate Properties [Line Items]
Investment commitment	7,828	58,973
Fair value of investments	$ 20,158	$ 72,786
Equity method investment, ownership percentage	49.90%	49.90%
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Investment Closing Date	5/14/2015	5/14/2015
Investment commitment	$ 0	$ 13,867
Fair value of investments	$ 1,638	$ 16,222
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Investment Closing Date	5/14/2015	5/14/2015
Investment commitment	$ 0	$ 14,849
Fair value of investments	$ 1,702	$ 16,588
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Investment Closing Date	9/25/2015	9/25/2015
Investment commitment	$ 0	$ 13,230
Fair value of investments	$ 4,211	$ 17,156
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Washington DC [Member]
Real Estate Properties [Line Items]
Investment Closing Date	4/15/2016	4/15/2016
Investment commitment	$ 0	$ 0
Fair value of investments	$ 3,727	$ 3,339
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | New Jersey [Member]
Real Estate Properties [Line Items]
Investment Closing Date	7/21/2016	7/21/2016
Investment commitment	$ 7,828	$ 7,828
Fair value of investments	8,880	$ 9,036
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Columbia [Member]
Real Estate Properties [Line Items]
Investment Closing Date		9/28/2016
Investment commitment		$ 9,199
Fair value of investments		10,445
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member]
Real Estate Properties [Line Items]
Funded Investment	7,471	56,962
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Funded Investment	0	13,114
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Funded Investment	0	14,519
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Funded Investment	0	12,899
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | Washington DC [Member]
Real Estate Properties [Line Items]
Funded Investment	0	0
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | New Jersey [Member]
Real Estate Properties [Line Items]
Funded Investment	7,471	7,357
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Funded Investment [Member] | Columbia [Member]
Real Estate Properties [Line Items]
Funded Investment		9,073
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member]
Real Estate Properties [Line Items]
Investment commitment	357	2,011
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Miami 1 [Member]
Real Estate Properties [Line Items]
Investment commitment	0	753
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Miami 2 [Member]
Real Estate Properties [Line Items]
Investment commitment	0	330
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Fort Lauderdale [Member]
Real Estate Properties [Line Items]
Investment commitment	0	331
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Washington DC [Member]
Real Estate Properties [Line Items]
Investment commitment	0	0
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | New Jersey [Member]
Real Estate Properties [Line Items]
Investment commitment	357	471
Development Property Investments [Member] | SL1 Venture [Member] | Loan Investments [Member] | Unfunded Commitment [Member] | Columbia [Member]
Real Estate Properties [Line Items]
Investment commitment		126
Eight Properties [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment commitment	$ 123,300	$ 81,400
Four Properties [Member] | SL1 Venture [Member] | Loan Investments [Member]
Real Estate Properties [Line Items]
Equity method investment, ownership percentage					100.00%	49.90%
Three Properties [Member] | SL1 Venture [Member] | Loan Investments [Member]
Real Estate Properties [Line Items]
Funded Investment							$ 8,100