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Schedule II Valuation and qualifying accounts (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for doubtful accounts - Tenant rent receivables      
Movement in valuation and qualifying accounts      
Balance at beginning of year $ 250,000 $ 100,000 $ 130,000
Additions (Decreases) charged to costs and expenses 128,000 178,000 78,000
Deductions (178,000) (28,000) (108,000)
Balance at end of year 200,000 250,000 100,000
Allowance for doubtful accounts - Straight-line rent receivable      
Movement in valuation and qualifying accounts      
Balance at beginning of year 50,000 50,000 50,000
Balance at end of year $ 50,000 $ 50,000 $ 50,000