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Note 3 - Accounting Policies - Available-for-sale Securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Amortized cost $ 67,405 $ 27,806
Gross unrealized gains
Gross unrealized losses (42) (10)
Estimated fair value 67,363 27,796
Debt Security, Corporate, US [Member]    
Amortized cost 16,202 13,003
Gross unrealized gains
Gross unrealized losses (42) (10)
Estimated fair value 16,160 12,993
United States Commercial Paper [Member]    
Amortized cost 17,402 4,929
Gross unrealized gains
Gross unrealized losses
Estimated fair value 17,402 4,929
Foreign Commercial Paper [Member]    
Amortized cost 33,801 9,874
Gross unrealized gains
Gross unrealized losses
Estimated fair value $ 33,801 $ 9,874