Stockholders' equity (Details) - Warrant [Member] - shares	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Shares
Outstanding, beginning balance	26,397,410	23,933,922
Expired or cancelled during the year	(1,928,333)	(80,310)
Exercised during the year	(12,270,853)	0
Issued in connection with issuance of convertible notes	1,116,671	1,483,340
Issued in connection with tender offer exercise	26,479,217	0
Outstanding, ending balance	39,794,112	25,336,952