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Note 3 - Accounting Policies - Available-for-sale Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Amortized cost $ 8,492 $ 37,741
Gross unrealized gains
Gross unrealized losses (17)
Estimated fair value 8,492 37,724
United States Commercial Paper [Member]    
Amortized cost 4,493 8,635
Gross unrealized gains
Gross unrealized losses
Estimated fair value 4,493 8,635
Domestic Corporate Debt Securities [Member]    
Amortized cost   12,048
Gross unrealized gains  
Gross unrealized losses   (9)
Estimated fair value   12,039
Foreign Commercial Paper [Member]    
Amortized cost 3,999 2,993
Gross unrealized gains
Gross unrealized losses
Estimated fair value $ 3,999 2,993
Foreign Corporate Debt Securities [Member]    
Amortized cost   14,065
Gross unrealized gains  
Gross unrealized losses   (8)
Estimated fair value   $ 14,057