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Stockholder's equity (deficit) - Undesignated preferred stock and Restricted common stock (Details) - $ / shares	1 Months Ended				6 Months Ended
	Jun. 30, 2019	Apr. 30, 2019	Oct. 31, 2017	Mar. 31, 2017	Jun. 30, 2019
Common Stock
Class of Stock
Issuance of restricted common stock upon early exercise of stock options (in shares)	487,933	487,933			487,933
Restricted stock
Class of Stock
Shares issued (in shares)				226,400
Stock options vesting period			2 years	4 years
Number of shares subject to be repurchased			75,467
Undesignated Preferred Stock
Class of Stock
Preferred stock, par value (in dollars per share)	$ 0.0001				$ 0.0001
Preferred stock, Shares Issued	0				0
Preferred stock, Shares Outstanding	0				0
Undesignated Preferred Stock | Maximum
Class of Stock
Preferred stock, shares authorized	10,000,000				10,000,000

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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