SELF-STORAGE INVESTMENT PORTFOLIO (Schedule of Cost Basis of Real Estate Investment) (Details) - USD ($)					3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 20, 2018	Feb. 02, 2018	Jan. 10, 2018	Aug. 09, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Funded principal	$ 24,754,000			$ 9,139,000	$ 365,479,000		$ 365,479,000		$ 213,069,000
Gain (Loss) on Investments							28,682,000		$ 26,267,000
Unrealized appreciation on investments	10,527,000			3,780,000	8,623,000	$ 4,289,000	12,943,000	$ 5,682,000
Cash consideration, inclusive of transaction costs	9,110,000			2,856,000
Assumed liabilities	1,258,000						1,258,000
Net property working capital acquired	216,000			52,000
Total cost basis	45,865,000			$ 15,827,000
Transaction cost incurred					$ 23,000		$ 23,000
Jacksonville 1 [Member]
Funded principal			$ 5,966,000
Unrealized appreciation on investments			2,947,000
Cash consideration, inclusive of transaction costs			2,625,000
Net property working capital acquired			95,000
Total cost basis			$ 11,633,000
Atlanta 1 [Member]
Funded principal		$ 8,084,000
Unrealized appreciation on investments		2,704,000
Cash consideration, inclusive of transaction costs		2,342,000
Net property working capital acquired		41,000
Total cost basis		13,171,000
Atlanta 2 [Member]
Funded principal		5,766,000
Unrealized appreciation on investments		2,900,000
Cash consideration, inclusive of transaction costs		2,960,000
Net property working capital acquired		40,000
Total cost basis		$ 11,666,000
Pittsburgh [Member]
Funded principal	4,938,000
Unrealized appreciation on investments	1,976,000
Cash consideration, inclusive of transaction costs	1,183,000
Assumed liabilities	1,258,000
Net property working capital acquired	40,000
Total cost basis	$ 9,395,000