Stockholders’ equity (deficit) (Details)	9 Months Ended
	Sep. 30, 2017 Vote $ / shares shares	Dec. 31, 2016 shares
Common stock
Number of votes for each shares held | Vote	1
Warrants
Number of shares of common stock into which warrants granted may be converted	129,491
Warrant exercise price per share (in dollars per share) | $ / shares	$ 0.05
Contractual life of warrants	10 years
Stock options.
Common stock
Number of shares reserved for future issuance	3,205,714	2,901,985
Warrants
Common stock
Number of shares reserved for future issuance	129,491	129,491
Series A-1 convertible preferred stock
Common stock
Number of shares reserved for future issuance		5,574,467
Series B-1 convertible preferred stock
Common stock
Number of shares reserved for future issuance		7,319,307
Series C-1 convertible preferred stock
Common stock
Number of shares reserved for future issuance		3,260,897
Convertible preferred stock, warrants and options
Common stock
Number of shares reserved for future issuance	3,335,205	19,186,147