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Note 4 - Fair Value Measurements - Financial Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Available-for-sale, current $ 122,124 $ 37,724
US Government Agencies Debt Securities [Member]    
Available-for-sale, current 24,971  
Domestic Corporate Debt Securities [Member]    
Available-for-sale, current 35,023 12,039
Foreign Corporate Debt Securities [Member]    
Available-for-sale, current 8,300 14,057
United States Commercial Paper [Member]    
Available-for-sale, current 36,897 8,635
Foreign Commercial Paper [Member]    
Available-for-sale, current 16,933 2,993
Fair Value, Measurements, Recurring [Member]    
Cash and money market funds 43,092 13,414
Assets, fair value 165,216 51,138
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member]    
Available-for-sale, current 24,971  
Fair Value, Measurements, Recurring [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale, current 35,023 12,039
Fair Value, Measurements, Recurring [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale, current 8,300 14,057
Fair Value, Measurements, Recurring [Member] | United States Commercial Paper [Member]    
Available-for-sale, current 36,897 8,635
Fair Value, Measurements, Recurring [Member] | Foreign Commercial Paper [Member]    
Available-for-sale, current 16,933 2,993
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Cash and money market funds 43,092 13,414
Assets, fair value 58,074 13,414
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Agencies Debt Securities [Member]    
Available-for-sale, current 14,982  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale, current  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale, current
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | United States Commercial Paper [Member]    
Available-for-sale, current  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale, current
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Cash and money market funds
Assets, fair value 107,142 37,724
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Agencies Debt Securities [Member]    
Available-for-sale, current 9,989  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale, current 35,023 12,039
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale, current 8,300 14,057
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | United States Commercial Paper [Member]    
Available-for-sale, current 36,897 8,635
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale, current 16,933 2,993
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Cash and money market funds
Assets, fair value  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Government Agencies Debt Securities [Member]    
Available-for-sale, current  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Domestic Corporate Debt Securities [Member]    
Available-for-sale, current  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Corporate Debt Securities [Member]    
Available-for-sale, current
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | United States Commercial Paper [Member]    
Available-for-sale, current  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Commercial Paper [Member]    
Available-for-sale, current