Summary of significant accounting policies - Antidilutive securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Anti-dilutive securities
Anti-dilutive securities	3,335,205	19,111,442	3,335,205	19,111,442
Series A-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		5,574,467		5,574,467
Series B-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		7,319,307		7,319,307
Series C-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		3,260,897		3,260,897
Warrants
Anti-dilutive securities
Anti-dilutive securities	129,491	129,491	129,491	129,491
Stock options.
Anti-dilutive securities
Anti-dilutive securities	3,205,714	2,827,280	3,205,714	2,827,280