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Stockholders' Equity - Early Exercise of Stock Options (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Nonvested shares (in shares) 47,882 90,977
Number of Shares    
Beginning balance (in shares) 90,977  
Ending balance (in shares) 47,882  
Stock options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Nonvested shares (in shares) 48,000 91,000
Other long-term liabilities $ 0.2 $ 0.4
Number of Shares    
Beginning balance (in shares) 91,000  
Early exercise of options (in shares) 0  
Vested (in shares) (43,000)  
Repurchased (in shares) 0  
Ending balance (in shares) 48,000