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General and administrative expenses - Summary of activity for restricted stock awards (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Restricted common stock      
Outstanding beginning of period 1,086,331 1,376,820 1,385,832
Outstanding end of period 545,500 1,086,331 1,376,820
Granted 567,000 900,000 824,000
Exercised [1] 1,085,331 1,132,988 833,012
Forfeited 22,500 57,501 0
Share options      
Outstanding 0 310,000 310,000
Forfeited 310,000 0 0
Weighted average exercise price      
Outstanding [2] $ 0 $ 9.64 $ 9.64
Forfeited $ 9.64 $ 0 $ 0
[1] Does not include shares in lieu of dividends.
[2] To be adjusted for dividends declared and paid subsequent to the respective grant dates.