INVESTMENT IN SELF-STORAGE REAL ESTATE VENTURE (Equity Method Investments) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Feb. 20, 2018	Feb. 02, 2018	Dec. 31, 2017	Aug. 09, 2017	Mar. 31, 2016
Real Estate Properties [Line Items]
Development property investments	$ 302,245			$ 228,233
Investment Commitment	673,090			523,821
Funded Investment	$ 365,479	$ 24,754		213,069	$ 9,139
Washington DC [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Percentage of completion of construction	40.00%
Atlanta 1 [Member]
Real Estate Properties [Line Items]
Funded Investment			$ 8,084
Atlanta 2 [Member]
Real Estate Properties [Line Items]
Funded Investment			$ 5,766
Funded Investment [Member]
Real Estate Properties [Line Items]
Funded Investment	$ 365,479			213,069
Unfunded Commitment [Member]
Real Estate Properties [Line Items]
Investment Commitment	307,611			$ 310,752
Loan investments [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Development property investments	$ 125,840
Funded Investment						$ 8,100
Loan investments [Member] | Miami 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	5/14/2015
Development property investments	$ 14,263
Loan investments [Member] | Miami 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	5/14/2015
Development property investments	$ 14,137
Loan investments [Member] | Fort Lauderdale [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	9/25/2015
Development property investments	$ 13,166
Loan investments [Member] | Washington DC [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	4/15/2016
Development property investments	$ 18,980
Loan investments [Member] | Atlanta 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	4/29/2016
Development property investments	$ 10,299
Loan investments [Member] | Jacksonville [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	7/19/2016
Development property investments	$ 11,594
Loan investments [Member] | New Jersey [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	7/21/2016
Development property investments	$ 3,525
Loan investments [Member] | Atlanta 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	8/15/2016
Development property investments	$ 9,119
Loan investments [Member] | Denver [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	8/25/2016
Development property investments	$ 12,335
Loan investments [Member] | Columbia [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	9/28/2016
Development property investments	$ 9,780
Loan investments [Member] | Raleigh [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Closing Date	12/22/2016
Development property investments	$ 8,642
Loan investments [Member] | Loans and Finance Receivables [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	123,273
Loan investments [Member] | Loans and Finance Receivables [Member] | Miami 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	13,867
Loan investments [Member] | Loans and Finance Receivables [Member] | Miami 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	14,849
Loan investments [Member] | Loans and Finance Receivables [Member] | Fort Lauderdale [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	13,230
Loan investments [Member] | Loans and Finance Receivables [Member] | Washington DC [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	17,269
Loan investments [Member] | Loans and Finance Receivables [Member] | Atlanta 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	10,223
Loan investments [Member] | Loans and Finance Receivables [Member] | Jacksonville [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	8,127
Loan investments [Member] | Loans and Finance Receivables [Member] | New Jersey [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	7,828
Loan investments [Member] | Loans and Finance Receivables [Member] | Atlanta 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	8,772
Loan investments [Member] | Loans and Finance Receivables [Member] | Denver [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	11,032
Loan investments [Member] | Loans and Finance Receivables [Member] | Columbia [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	9,199
Loan investments [Member] | Loans and Finance Receivables [Member] | Raleigh [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	8,877
Loan investments [Member] | Funded Investment [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	106,134
Loan investments [Member] | Funded Investment [Member] | Miami 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	11,829
Loan investments [Member] | Funded Investment [Member] | Miami 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	12,704
Loan investments [Member] | Funded Investment [Member] | Fort Lauderdale [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	11,211
Loan investments [Member] | Funded Investment [Member] | Washington DC [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	16,426
Loan investments [Member] | Funded Investment [Member] | Atlanta 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	9,304
Loan investments [Member] | Funded Investment [Member] | Jacksonville [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	7,378
Loan investments [Member] | Funded Investment [Member] | New Jersey [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	3,493
Loan investments [Member] | Funded Investment [Member] | Atlanta 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	7,982
Loan investments [Member] | Funded Investment [Member] | Denver [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	9,998
Loan investments [Member] | Funded Investment [Member] | Columbia [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	8,566
Loan investments [Member] | Funded Investment [Member] | Raleigh [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Funded Investment	7,243
Loan investments [Member] | Unfunded Commitment [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	17,139
Loan investments [Member] | Unfunded Commitment [Member] | Miami 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	2,038
Loan investments [Member] | Unfunded Commitment [Member] | Miami 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	2,145
Loan investments [Member] | Unfunded Commitment [Member] | Fort Lauderdale [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	2,019
Loan investments [Member] | Unfunded Commitment [Member] | Washington DC [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	843
Loan investments [Member] | Unfunded Commitment [Member] | Atlanta 1 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	919
Loan investments [Member] | Unfunded Commitment [Member] | Jacksonville [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	749
Loan investments [Member] | Unfunded Commitment [Member] | New Jersey [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	4,335
Loan investments [Member] | Unfunded Commitment [Member] | Atlanta 2 [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	790
Loan investments [Member] | Unfunded Commitment [Member] | Denver [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	1,034
Loan investments [Member] | Unfunded Commitment [Member] | Columbia [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	633
Loan investments [Member] | Unfunded Commitment [Member] | Raleigh [Member] | SL1 Venture [Member]
Real Estate Properties [Line Items]
Investment Commitment	$ 1,634