Nature of the business and basis of presentation (Details) $ / shares in Units, $ in Thousands			3 Months Ended						9 Months Ended
	Jun. 24, 2019 USD ($) $ / shares shares	Jun. 06, 2019	Sep. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares
Subsidiary, Sale of Stock
Common stock issued (in shares)			28,558,653						28,558,653		238,986
Authorized common stock (in shares)			150,000,000						150,000,000		75,000,000
Par value common stock (in dollars per share) | $ / shares			$ 0.0001						$ 0.0001		$ 0.0001
Reverse stock split of the common stock		3.07418
Net loss | $			$ 15,554	$ 7,228	$ 5,362	$ 11,148	$ 8,673	$ 421	$ 28,144	$ 20,242
Accumulated deficit | $			114,709						114,709		$ 86,565
Cash and cash equivalents | $			$ 147,835						$ 147,835		$ 75,975
IPO
Subsidiary, Sale of Stock
Common stock issued (in shares)	6,612,500
Public offering price (in dollars per share) | $ / shares	$ 16.00
Proceeds from issuing shares | $	$ 98,394
Offering costs | $	$ 2,942
Conversion of convertible preferred stock into common stock	21,056,136
Authorized common stock (in shares)	150,000,000
Par value common stock (in dollars per share) | $ / shares	$ 0.0001
Authorized Preferred stock (in shares)	10,000,000
Par value preferred stock (in dollars per share) | $ / shares	$ 0.0001
Over allotment
Subsidiary, Sale of Stock
Proceeds from issuing shares | $	$ 98,394
Over allotment | Maximum
Subsidiary, Sale of Stock
Common stock issued (in shares)	862,500