Redeemable convertible preferred stock (Details) $ / shares in Units, $ in Millions		1 Months Ended			12 Months Ended
	Dec. 04, 2017	Nov. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2017 shares
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					47,015,449
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.001
Common stock
Redeemable convertible preferred stock prior to IPO
Preferred stock conversion ratio	0.311
Initial Public Offering
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares						5,000,000
Series A-1 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					3,972,415
Series A-2 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					10,492,027
Series A-3 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					2,000,000
Preferred stock, shares issued				700,000
Purchase price | $ / shares				$ 0.001
Fair value of preferred warrants | $				$ 2.1
Series B Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					6,186,594
Series C Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					9,247,089
Preferred stock, shares issued		31,283
Fair value of preferred warrants | $		$ 0.1
Warrants to purchase shares in cashless		22,809
Milestone Amount | $					$ 10.0
Series C Preferred Stock ,purchase price $3.3299 per share
Redeemable convertible preferred stock prior to IPO
Preferred stock, shares issued		8,474
Purchase price | $ / shares		$ 3.3299
Series C-1 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					544,332
Series D Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					12,459,090
Series D-1 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares					2,113,902
Preferred stock, shares issued			2,113,902
Purchase price | $ / shares			$ 4.0210
Cash proceeds | $			$ 8.4