RELATED PARTY TRANSACTIONS (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Development property investments	$ 302,245		$ 302,245		$ 228,233
Base management fee, annual rate			1.50%
Management fee, description			The Manager computes the base management fee within 30 days after the end of the fiscal quarter with respect to which such installment is payable and promptly delivers such calculation to the Company's Board of Directors. The amount of the installment shown in the calculation is due and payable no later than the date which is five business days after the date of delivery of such computation to the Board of Directors. The calculation generally will be reviewed by the Board of Directors at their regularly scheduled quarterly board meeting
Percentage of base management fee	0.375%		0.375%
Expenses reimbursable to Manager	$ 900	$ 600	$ 1,800	$ 1,500
Management fee expense	1,582	707	$ 2,886	1,337
Incentive fee, description			Notwithstanding application of the incentive fee formula, no incentive fee shall be paid with respect to any fiscal quarter unless cumulative annual stockholder total return for the four most recently completed fiscal quarters is greater than 8%. Any computed incentive fee earned but not paid because of the foregoing hurdle will accrue until such 8% cumulative annual stockholder total return is achieved
Income (Loss) from equity method investments	435	595	$ 985	1,017
Outstanding fees due to Manager	1,887		1,887		1,484
Investment in and advances to self-storage real estate venture	14,846		14,846		13,856
SL1 Venture [Member]
Related Party Transaction [Line Items]
Income (Loss) from equity method investments	400	1,000	600	1,000
Investment in and advances to self-storage real estate venture	14,800		14,800		13,900
Equity Method Investments [Member]
Related Party Transaction [Line Items]
Development property investments	366,000		366,000		$ 215,900
Accumulated change in fair value from equity method investment and interest income realized	$ 14,700	$ 6,100	$ 23,000	$ 8,700