Note 3 - Real Estate - Summary of Geographic Concentration of Properties (Parenthetical) (Details) - Rental Income [Member] - Geographic Concentration Risk [Member] - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Concentration Risk [Line Items]
Revenues	$ 3,938,000	$ 4,146,000
Ground Rental Revenue [Member] \| Maximum [Member]
Concentration Risk [Line Items]
Revenues	$ 100,000	$ 100,000

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details