Common stock - Narrative (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 vote $ / shares shares	Dec. 31, 2016 shares	Dec. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized (shares)	100,000,000		100,000,000
Common stock, par value (in usd per share) | $ / shares	$ 0.0001		$ 0.0001
Common stock, number of votes per share | vote	1
Restricted stock | Non-employee Consultants and Advisers
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted common stock, shares sold (shares)		18,194
Restricted common stock, shares outstanding (shares)	17,360