Common stock (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 14, 2012	Dec. 31, 2011
Common stock
Number of votes to which holders of common shares are entitled for each share held at all general meetings	1
Number of common shares authorized	250,000,000	250,000,000	250,000,000	250,000,000	80,000,000
Number of preferred shares authorized	10,000,000	10,000,000	10,000,000	10,000,000	4,000,000
Common stock
Common stock issued
Restricted stock grants (in shares)	(243)	860,739
Employee option exercises (in shares)	564,077	304,769	488,513
Warrant exercises (in shares)	79,677	213,123	917,602
Consideration shares upon acquisition of BPS (in shares)			488,337
Total common stock issued	643,511	1,378,631	1,894,452