Redeemable convertible preferred stock (Details) $ / shares in Units, $ in Millions					1 Months Ended							2 Months Ended	12 Months Ended
	Dec. 07, 2017 shares	Dec. 04, 2017	Dec. 31, 2015	Nov. 30, 2015	Nov. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares	Feb. 29, 2016 USD ($) $ / shares shares	May 31, 2015 USD ($) $ / shares shares	Jan. 31, 2015 USD ($) $ / shares shares	Jul. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 shares	Mar. 31, 2017 shares	Nov. 18, 2016 shares	Jan. 29, 2016 shares	Jan. 01, 2016 shares
Redeemable convertible preferred stock prior to IPO
Preferred stock, par value (in dollars per share) | $ / shares													$ 0.001
Preferred stock conversion ratio		0.311
Conversion shares of common stock	45,593,028
Common stock
Redeemable convertible preferred stock prior to IPO
Preferred stock conversion ratio		0.311											3.214
Initial Public Offering
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares														5,000,000
Initial Public Offering | Common stock
Redeemable convertible preferred stock prior to IPO
Preferred stock conversion ratio	3.214
Minimum percentage of outstanding shares to be converted													60.00%
Conversion shares of common stock	14,185,744
Initial Public Offering | Common stock | Minimum
Redeemable convertible preferred stock prior to IPO
Purchase price | $ / shares													$ 5.00
Conversion, threshold gross proceeds | $													$ 40.0
Stock Purchase Agreement | bioMerieux
Redeemable convertible preferred stock prior to IPO
Milestone Amount | $													10.0
Number of instalments			3	1
Milestone Payment1 | bioMerieux
Redeemable convertible preferred stock prior to IPO
Milestone Amount | $													5.0
Milestone Payment2 | bioMerieux
Redeemable convertible preferred stock prior to IPO
Milestone Amount | $													3.0
Milestone Payment3 | bioMerieux
Redeemable convertible preferred stock prior to IPO
Milestone Amount | $													$ 2.0
Preferred stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													47,015,449
Series A Preferred Stock
Redeemable convertible preferred stock prior to IPO
Fair value of preferred warrants | $							$ 2.1		$ 3.9
Number of annual instalments													3
Series A-1 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													3,972,415
Annual rate on cumulative dividends | $ / shares													$ 0.08
Cumulative accrued dividends | $													$ 3.3
Liquidation preference per share | $ / shares													$ 1.0416667
Votes per share													1
Eligibility to elect number of directors													3
Number of annual instalments													3
Conversion shares of common stock	3,972,415
Series A-2 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													10,492,027
Warrants to purchase shares																		64,441
Annual rate on cumulative dividends | $ / shares													$ 0.08
Cumulative accrued dividends | $													$ 7.8
Liquidation preference per share | $ / shares													$ 1.0416667
Votes per share													1
Eligibility to elect number of directors													3
Number of annual instalments													3
Conversion shares of common stock	10,427,586
Series A-3 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													2,000,000
Warrants to purchase shares							700,000		1,300,000					700,000		700,000		1,300,000
Purchase price | $ / shares							$ 0.001		$ 0.001
Liquidation preference per share | $ / shares													$ 2.00
Votes per share													1
Number of annual instalments													3
Conversion shares of common stock	2,000,000
Series B Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													6,186,594
Warrants to purchase shares												397,530						562,488
Warrants to purchase shares in cashless												76,700
Fair value of preferred warrants | $												$ 1.4
Annual rate on cumulative dividends | $ / shares													$ 0.16
Cumulative accrued dividends | $													$ 5.7
Liquidation preference per share | $ / shares													$ 2.00
Votes per share													1
Eligibility to elect number of directors													3
Number of annual instalments													3
Conversion shares of common stock	6,021,636
Series B Preferred Stock, purchase price $0.001 per share
Redeemable convertible preferred stock prior to IPO
Warrants to purchase shares												312,500
Purchase price | $ / shares												$ 0.001
Series B Preferred Stock, purchase price $2.00 per share
Redeemable convertible preferred stock prior to IPO
Warrants to purchase shares												8,330
Purchase price | $ / shares												$ 2.00
Series C Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													9,247,089
Warrants to purchase shares					31,283									111,114			57,810	226,733
Warrants to purchase shares in cashless					22,809
Fair value of preferred warrants | $					$ 0.1
Annual rate on cumulative dividends | $ / shares													$ 0.2664
Cumulative accrued dividends | $													$ 9.5
Liquidation preference per share | $ / shares													$ 3.3299
Votes per share													1
Eligibility to elect number of directors													1
Number of annual instalments													3
Conversion shares of common stock	8,092,895
Series C Preferred Stock | Stock Purchase Agreement | bioMerieux
Redeemable convertible preferred stock prior to IPO
Purchase price | $ / shares										$ 3.3299	$ 3.3299
Preferred stock, shares issued										600,618	1,501,546
Cash proceeds | $										$ 2.0	$ 5.0
Series C Preferred Stock ,purchase price $3.3299 per share
Redeemable convertible preferred stock prior to IPO
Warrants to purchase shares					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
Annual rate on cumulative dividends | $ / shares													$ 0.2664
Cumulative accrued dividends | $													$ 0.7
Liquidation preference per share | $ / shares													$ 3.3299
Votes per share													0
Number of annual instalments													3
Conversion shares of common stock	544,332
Series D Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													12,459,090
Warrants to purchase shares															38,828
Purchase price | $ / shares								$ 3.67
Preferred stock, shares issued								12,420,262
Cash proceeds | $								$ 45.4
Annual rate on non-cumulative dividends | $ / shares													$ 0.2936
Liquidation preference per share | $ / shares													$ 3.67
Votes per share													1
Number of annual instalments													3
Conversion shares of common stock	12,420,262
Series D-1 Preferred Stock
Redeemable convertible preferred stock prior to IPO
Preferred stock, authorized shares													2,113,902
Purchase price | $ / shares						$ 4.021
Preferred stock, shares issued						2,113,902
Cash proceeds | $						$ 8.4
Annual rate on non-cumulative dividends | $ / shares													$ 0.3217
Liquidation preference per share | $ / shares													$ 4.021
Votes per share													1
Number of annual instalments													3
Conversion shares of common stock	2,113,902