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Share-Based Compensation - Stock Options Activity (Details) - 2021 Equity Incentive Plan - Stock options - shares
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Stock Options    
Options outstanding at beginning of period 1,090,300 1,529,800
Granted 0 0
Exercised (114,000) (255,550)
Forfeited 0 (2,850)
Options outstanding at end of period 976,300 1,271,400
Options exercisable at end of period 938,800 1,124,800