Note 8 - Rental Income Under Operating Leases (Details Textual)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total Revenue [Member] \| Customer Concentration Risk [Member] \| City of New York [Member]
Concentration Risk, Percentage	19.00%	20.00%	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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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