Schedule II Valuation and qualifying accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts - Tenant rent receivables
Movement in valuation and qualifying accounts
Balance at beginning of year	$ 100,000	$ 130,000	$ 325,000
Additions (Decreases) charged to costs and expenses	178,000	78,000	(106,000)
Deductions	(28,000)	(108,000)	(89,000)
Balance at end of year	250,000	100,000	130,000
Allowance for doubtful accounts - Straight-line rent receivable
Movement in valuation and qualifying accounts
Balance at beginning of year	50,000	50,000	162,000
Additions (Decreases) charged to costs and expenses			(112,000)
Deductions			(112,000)
Balance at end of year	$ 50,000	$ 50,000	$ 50,000