Schedule II Valuation and qualifying accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for doubtful accounts - Tenant rent receivables
Movement in valuation and qualifying accounts
Balance at beginning of year	$ 300,000	$ 200,000	$ 250,000
Additions (Decreases) charged to costs and expenses	340,000	213,000	128,000
Deductions	(135,000)	(113,000)	(178,000)
Balance at end of year	505,000	300,000	200,000
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
Deductions	$ (50,000)
Balance at end of year		$ 50,000	$ 50,000