Note 6 - Financial Instruments and Fair Value (Details Textual)	1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 28, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2011 USD ($)	Mar. 31, 2018 USD ($)
Asset Impairment Charges, Total			$ 469,000			$ 427,000	$ 1,000,000
Impairment of Real Estate				$ 7,500,000
Number of Real Estate Properties Sold During Period	1		3	4
Real Estate Held-for-sale	$ 1,900,000		$ 2,200,000	$ 6,100,000
Number of Real Estate Properties Put Back into Service		1
Real Estate Previously Held-for-sale Put Back Into Service		$ 1,400,000
Auction Rate Securities [Member]
Debt Securities, Available-for-sale, Total			6,400,000					$ 6,400,000
Debt Securities, Available-for-sale, Amortized Cost, Total			6,400,000					$ 6,400,000
Auction Rate Securities Redeemed			325,000	$ 450,000	$ 275,000	$ 150,000
Derivative, Basis Spread on Variable Rate								2.25%
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Debt Securities, Available-for-sale, Total			$ 6,400,000					$ 6,400,000