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General & Administrative Expenses- Shares held by executives and directors (Details) - shares
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
General & Administrative Expenses [Abstract]      
Number of shares held by Executives and Directors as a group (in shares) [1] 2,303,011 3,615,221 3,155,503
Number of restricted stock subject to vesting conditions (in shares) 619,998 440,000 465,000
Base salary entitled to executive officer for termination of employment 18 months    
[1] Includes 619,998 (2019: 440,000, 2018: 465,000) shares of restricted stock subject to vesting conditions.