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Stock-Based Compensation Expense and Employee Benefit Plans - Employee Stock Purchase Plan (Details) - 2018 ESPP - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee stock ownership plan (ESOP), compensation expense $ 2.2 $ 4.7
Share-based compensation arrangement by share-based payment award, shares issued in period 977,508  
Number of additional shares authorized (in shares) 1,902,572  
Number of common stock reserved for issuance (in shares) 3,512,465  
Unrecognized stock-based compensation cost $ 2.1 $ 3.7
Expense expected to be recognized over a weighted-average period 2 months 12 days 7 months 6 days