Note 9 - Financial Instruments and Fair Value (Details Textual)	3 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2011 USD ($)
Asset Impairment Charges		$ 469,000		$ 8,247,000		$ 427,000	$ 1,000,000
Impairment of Real Estate				$ 7,500,000
Number of Real Estate Properties Sold During Period			3	4
Real Estate Held-for-sale			$ 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]
Available-for-sale Securities			6,375,000	6,231,000
Available-for-sale Securities, Amortized Cost Basis			$ 6,375,000	6,700,000
Auction Rate Securities Redeemed		$ 325,000		$ 450,000	$ 275,000	$ 150,000
Derivative, Basis Spread on Variable Rate			2.25%