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Note 4 - Fair Value Measurements - Financial Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Available-for-sale securities $ 8,492 $ 37,724
United States Commercial Paper [Member]    
Available-for-sale securities 4,493 8,635
Domestic Corporate Debt Securities [Member]    
Available-for-sale securities   12,039
Foreign Commercial Paper [Member]    
Available-for-sale securities 3,999 2,993
Foreign Corporate Debt Securities [Member]    
Available-for-sale securities   14,057
Fair Value, Measurements, Recurring [Member]    
Total 71,004 49,217
Fair Value, Measurements, Recurring [Member] | United States Commercial Paper [Member]    
Available-for-sale securities 4,493 8,635
Fair Value, Measurements, Recurring [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale securities   12,039
Fair Value, Measurements, Recurring [Member] | Foreign Commercial Paper [Member]    
Available-for-sale securities 3,999 2,993
Fair Value, Measurements, Recurring [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale securities   14,057
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Total 62,512 11,493
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | United States Commercial Paper [Member]    
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale securities  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale securities  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Total 8,492 37,724
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | United States Commercial Paper [Member]    
Available-for-sale securities 4,493 8,635
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale securities   12,039
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale securities 3,999 2,993
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale securities   14,057
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Total
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | United States Commercial Paper [Member]    
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale securities  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale securities  
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]    
Money market funds 62,512 11,493
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]    
Money market funds 62,512 11,493
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]    
Money market funds
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]    
Money market funds