Subsequent events (Details) - Expiration Date, July 2019 [Member] - $ / shares		6 Months Ended
	Jul. 24, 2019	Jun. 30, 2019
Warrants and Rights [Abstract]
Expiration date		Jul. 24, 2019
Minimum [Member]
Warrants and Rights [Abstract]
Exercise price (in dollars per share)		$ 3.75
Maximum [Member]
Warrants and Rights [Abstract]
Exercise price (in dollars per share)		$ 12.25
Subsequent Event [Member]
Warrants and Rights [Abstract]
Expiration date	Jul. 24, 2019
Warrants expired (in shares)	(1,239,769)
Subsequent Event [Member] | Minimum [Member]
Warrants and Rights [Abstract]
Exercise price (in dollars per share)	$ 3.75
Subsequent Event [Member] | Maximum [Member]
Warrants and Rights [Abstract]
Exercise price (in dollars per share)	$ 12.25