Basic and diluted earnings per share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Basic and diluted earnings per share
Total potential dilutive common shares	11,196,892	9,072,746
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	7,570,000	5,286,254
Stock options | 2014 Plan
Basic and diluted earnings per share
Total potential dilutive common shares	2,707,538	2,666,188
Stock options | 2012 Plan
Basic and diluted earnings per share
Total potential dilutive common shares	32,567	182,202
Non-vested and earned RSUs and PSUs
Basic and diluted earnings per share
Total potential dilutive common shares	849,612	900,927