Convertible preferred stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			9 Months Ended	17 Months Ended	42 Months Ended
	Jul. 03, 2017	Jul. 31, 2017	Jun. 30, 2016	Feb. 28, 2015	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Convertible preferred stock.
Convertible preferred stock converted into commons stock	16,154,671	16,154,671
Series A-1 convertible preferred stock
Convertible preferred stock.
Issuance of convertible preferred stock (in shares)							25,085,153
Issue price (in dollars per share)							$ 1.0763
Net proceeds from sale of convertible preferred stock							$ 26,336
Series B-1 convertible preferred stock
Convertible preferred stock.
Issuance of convertible preferred stock (in shares)			23,526,368	9,410,551
Issue price (in dollars per share)			$ 1.0763	$ 1.0763		$ 1.0763
Net proceeds from sale of convertible preferred stock					$ 25,272	$ 35,232
Series C-1 convertible preferred stock
Convertible preferred stock.
Issuance of convertible preferred stock (in shares)			14,674,062
Issue price (in dollars per share)			$ 2.25568			$ 2.25568
Net proceeds from sale of convertible preferred stock			$ 32,882		$ 32,882