Basic and diluted earnings per share (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic and diluted earnings per share
Total potential dilutive common shares	11,032,239	7,198,235
Warrants
Basic and diluted earnings per share
Total potential dilutive common shares	37,175	37,175
Warrants | BMS
Basic and diluted earnings per share
Total potential dilutive common shares	6,950,000	3,587,333
Stock options | 2014 Plan
Basic and diluted earnings per share
Total potential dilutive common shares	2,876,475	2,605,443
Stock options | 2012 Plan
Basic and diluted earnings per share
Total potential dilutive common shares	63,818	348,032
Non-vested and earned RSUs and PSUs
Basic and diluted earnings per share
Total potential dilutive common shares	1,104,771	620,252