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Share-Based Compensation - Fair Value of Shares Vested and Weighted Average Estimated Fair Value of Shares Granted (Details) - Stock Options - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total fair value of shares vested $ 407 $ 515
Weighted average estimated fair value of shares granted $ 1.08 $ 2.12