Common stock (Details)	12 Months Ended
	Dec. 31, 2015 item shares	Dec. 31, 2014 shares	Dec. 31, 2013 shares
Common stock
Number of votes to which holders of common shares are entitled for each share held at all general meetings | item	1
Common Stock
Common stock issued
Restricted stock grants (in shares)	752,835	789,394
Restricted stock cancellations (in shares)	(1,025,559)	(789,637)
Employee option exercises (in shares)	783,114	564,077	304,769
Warrant exercises (in shares)		79,677	213,123
Total common stock issued	510,390	643,511