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Note 7 - Stockholders' Equity and Stock-based Compensation Expense (Details Textual) - USD ($)
12 Months Ended
Nov. 22, 2022
May 05, 2022
Feb. 12, 2021
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
May 01, 2023
Mar. 31, 2020
Proceeds from Issuance of Common Stock       $ 0 $ 47,329,000 $ 189,825,000    
Common Stock, Par or Stated Value Per Share (in dollars per share)       $ 0.001 $ 0.001      
Stock Options Exercised Net Settled Satisfaction Of Exercise Price (in shares)       101,058        
Share-Based Payment Arrangement, Option [Member]                
Share-Based Compensation Arrangement by Share-Based Payment Award, Fair Value Assumptions, Expected Dividend Rate       0.00% 0.00% 0.00%    
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount       $ 24,200,000        
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)       2 years 9 months 18 days        
Performance Shares [Member]                
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount         $ 101,000      
Share-Based Compensation Arrangement by Share-Based Payment Award, Non-Option Equity Instruments, Expirations (in shares)         57,143      
The 2018 Equity Incentive Plan [Member]                
Share-Based Compensation Arrangement by Share-Based Payment Award, Expiration Period (Year)   10 years            
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares)   5,000,000            
Common Stock, Par or Stated Value Per Share (in dollars per share)   $ 0.001            
The 2022 Registered Direct Offering [Member]                
Stock Issued During Period, Shares, New Issues (in shares) 1,666,667              
Shares Issued, Price Per Share (in dollars per share) $ 30              
Proceeds from Issuance of Common Stock $ 47,300,000              
The 2021 Registered Direct Offering [Member]                
Stock Issued During Period, Shares, New Issues (in shares)     4,081,633          
Shares Issued, Price Per Share (in dollars per share)     $ 49          
Proceeds from Issuance of Common Stock     $ 189,800,000          
At-the-market Common Stock Offering [Member]                
Stock Issued During Period, Shares, New Issues (in shares)       0        
Equity Offering, Maximum Amount             $ 200,000,000 $ 100,000,000
Equity Offering, Percentage of Commission             3.00%