Note 7 - Rental Income Under Operating Leases (Details Textual)	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Total Revenue [Member] \| Customer Concentration Risk [Member] \| City of New York [Member]
Concentration Risk, Percentage	19.00%	19.00%	19.00%	19.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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