Stock-based compensation - Summary of stock option activity (Details) - USD ($)		6 Months Ended	12 Months Ended
	Jun. 14, 2018	Jun. 30, 2019	Dec. 31, 2018
Shares
Shares outstanding, Beginning Balance (shares)		2,320,981
Shares granted (shares)	654,544	0
Shares exercised (shares)		0
Shares forfeited (shares)		0
Shares expired (shares)		(23,945)
Shares outstanding, Ending Balance (shares)		2,297,036	2,320,981
Shares exercisable (shares)		1,346,425
Shares vested or expected to vest (shares)		2,297,036
Weighted-Average Exercise Price
Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share)		$ 4.10
Shares granted, weighted-average exercise price (in usd per share)		0.00
Shares exercised, weighted-average exercise price (in usd per share)		0.00
Shares forfeited, weighted-average exercise price (in usd per share)		0.00
Shares expired, weighted-average exercise price (in usd per share)		4.41
Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share)		4.10	$ 4.10
Shares exercisable, weighted-average exercise price (in usd per share)		5.11
Shares vested or expected to vest, weighted-average exercise price (in usd per share)		$ 4.10
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term (in years)		7 years 1 month 10 days	7 years 7 months 10 days
Exercisable, weighted-average remaining contractual term (in years)		6 years 18 days
Vested or expected to vest, weighted-average remaining contractual term (in years)		7 years 1 month 10 days
Outstanding, aggregate intrinsic value		$ 0	$ 0
Exercisable, aggregate intrinsic value		0
Vested or expected to vest, aggregate intrinsic value		$ 0