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Note 7 - Marketable Securities, Available-for-sale - Fair Values by Type of Security (Details) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
Gross unrealized losses $ (172,000)  
Estimated fair value 24,529,000  
Municipal Bond and Metropolitan District Securities [Member]    
Amortized cost 24,994,000 $ 18,563,000
Gross unrealized losses (465,000) (465,000)
Estimated fair value $ 24,529,000 $ 18,098,000