| Restricted common stock | Share options | Weighted average exercise price ** | ||||||||||||
| Outstanding at December 31, 2013 | 493,523 | 310,965 | 9.64 | |||||||||||
| Granted | 750,000 | |||||||||||||
| Exercised* | 324,008 | |||||||||||||
| Forfeited | ||||||||||||||
| Outstanding at December 31, 2014 | 919,515 | 310,965 | 9.64 | |||||||||||
| Outstanding at December 31, 2014 | 919,515 | 310,965 | 9.64 | |||||||||||
| Granted | 850,000 | |||||||||||||
| Exercised* | 383,683 | |||||||||||||
| Forfeited | 965 | |||||||||||||
| Outstanding at December 31, 2015 | 1,385,832 | 310,000 | $ | 9.64 | ||||||||||
| Outstanding at December 31, 2015 | 1,385,832 | 310,000 | 9.64 | |||||||||||
| Granted | 824,000 | |||||||||||||
| Exercised* | 833,012 | |||||||||||||
| Forfeited | - | |||||||||||||
| Outstanding at December 31, 2016 | 1,376,820 | 310,000 | $ | 9.64 | ||||||||||
| Outstanding at December 31, 2016 | 1,376,820 | 310,000 | 9.64 | |||||||||||
| Granted | 900,000 | |||||||||||||
| Exercised* | 1,132,988 | |||||||||||||
| Forfeited | 57,501 | |||||||||||||
| Outstanding at December 31, 2017 | 1,086,331 | 310,000 | $ | 9.64 | ||||||||||