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Share-Based Compensation - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 12 Months Ended
Oct. 31, 2019
Aug. 01, 2019
Apr. 30, 2019
Oct. 31, 2018
Dec. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2021
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Stock options outstanding (in shares)         4,218,278 4,218,278 5,170,976 5,120,293   4,207,029
Restricted stock units                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued, unvested (in shares)         275,989 275,989 149,004 0    
Share-based compensation expense           $ 9,800,000 $ 3,700,000 $ 0    
Unrecognized compensation expense, period of recognition           1 year 9 months 18 days        
Unrecognized share-based compensation expense         $ 14,900,000 $ 14,900,000        
Vested (in shares)           99,970 23,552      
Granted (in shares)           244,752 173,196      
Restricted stock price (in dollars per share)           $ 109.61 $ 126.29      
Fair value per share (in dollars per share)         $ 114.99 $ 114.99 $ 132.73 $ 0    
Restricted stock units | Employee                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period           4 years 4 years      
Restricted stock units | Nonemployee | Director                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period           12 months        
Restricted stock units | First Anniversary                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting percentage           25.00% 25.00%      
Restricted stock units | Thereafter                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period           3 years        
Employee Stock Options                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period               10 years    
Share-based compensation expense           $ 13,100,000 $ 6,300,000 $ 1,500,000    
Unrecognized compensation expense         $ 16,400,000 $ 16,400,000        
Unrecognized compensation expense, period of recognition           2 years        
Employee Stock Options | Employee                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period           48 months        
Employee Stock Options | Employee | Executive Chair                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period 24 months 48 months                
Employee Stock Options | First Anniversary                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting percentage           25.00% 25.00%      
Employee Stock Options | Thereafter                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period           3 years 3 years      
Common Stock                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Unrecognized compensation expense, period of recognition             8 months 12 days      
Unrecognized share-based compensation expense             $ 2,500,000      
Share-based compensation expense accrued, not issued           $ 3,000,000.0 1,000,000.0      
Share-based compensation expense accrued, amount reclassified to APIC           3,000,000.0        
Share-based compensation liability         $ 1,000,000.0 $ 1,000,000.0 $ 1,000,000.0      
Vested (in shares)         23,666          
Fair value of shares vested         $ 3,000,000.0          
Restricted Stock                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued, unvested (in shares)         12,184 12,184 88,988 100,127   0
Vesting period     24 months              
Share-based compensation expense           $ 1,400,000 $ 1,800,000 $ 700,000    
Unrecognized share-based compensation expense         $ 200,000 $ 200,000        
Vested (in shares)           76,804 87,239 35,664    
Granted (in shares)       135,791   0 99,433 135,791    
Restricted stock price (in dollars per share)           $ 0 $ 20.02 $ 17.03    
Purchase price (in dollars per share)     $ 0.01 $ 0.02            
Fair value per share (in dollars per share)       $ 17.03 $ 20.02 $ 20.02 $ 19.49 $ 17.03   $ 0
Shares of restricted stock purchased by nonemployees (in shares)         12,184 12,184        
Weighted average remaining contractual life           3 months 18 days 1 year 2 months 12 days 1 year 7 months 6 days    
Restricted Stock | Minimum                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period       12 months            
Restricted Stock | Maximum                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Vesting period       24 months            
2018 Plan                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares authorized for issuance (in shares)         16,626,877 16,626,877        
Additional shares authorized annually (in shares)           2,144,521        
Percent of common stock outstanding, per year           4.00%        
Shares issued (in shares)           7,127,079 5,864,738      
2018 Plan | Subsequent Event                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares authorized for issuance (in shares)                 18,771,398  
2018 Plan | Restricted stock units                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares issued, unvested (in shares)         275,989 275,989        
2018 ESPP | Employee Stock                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares authorized for issuance (in shares)         1,340,325 1,340,325        
Additional shares authorized annually (in shares)           536,130        
Percent of common stock outstanding, per year           1.00%        
Period of additional shares authorized           10 years        
2018 ESPP | Employee Stock | Subsequent Event                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Shares authorized for issuance (in shares)                 1,876,455