Stockholders' equity (Details) - Warrant [Member] - shares	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Shares
Outstanding, beginning balance	26,397,410	23,933,922
Expired or cancelled during the year	(546,667)	(73,530)
Exercised during the year	(12,270,853)	0
In connection with issuance of convertible notes	0	750,004
In connection with tender offer exercise	26,479,217	0
Outstanding, ending balance	40,059,107	24,610,396