Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	12 Months Ended
	Jan. 19, 2018	Oct. 31, 2017	Jul. 31, 2016	Jan. 31, 2016	Mar. 31, 2017	Dec. 31, 2018	Dec. 21, 2018	Dec. 31, 2017	Jun. 25, 2018	Dec. 31, 2016
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						63,491,857		34,972,535		22,380,952
Redeemable convertible preferred stock, par value						$ 0.0001		$ 0.0001		$ 0.0001
Redeemable convertible preferred stock, shares issued and sold								34,783,832
Proceeds from issuance of redeemable convertible preferred stock						$ 58,257		$ 17,415
IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, sales price per share									$ 19.00
Preferred stock converted into common stock							15,320,213
Series Seed Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						3,333,333		3,333,333		3,333,333
Redeemable convertible preferred stock, shares issued and sold				3,333,333				3,333,333
Redeemable convertible preferred stock, sales price per share				$ 0.45
Proceeds from issuance of redeemable convertible preferred stock				$ 1,398
Redeemable convertible preferred stock, net of issuance costs				$ 102
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						31,639,202		31,639,202		19,047,619
Redeemable convertible preferred stock, shares issued and sold		17,612,279	5,714,286		4,403,070			31,450,499
Redeemable convertible preferred stock, sales price per share		$ 0.7949	$ 1.3125		$ 0.7949
Proceeds from issuance of redeemable convertible preferred stock		$ 13,963	$ 7,297		$ 3,452
Redeemable convertible preferred stock, net of issuance costs		$ 37	$ 203		$ 48
Issuance of redeemable convertible preferred stock dividend					3,720,864
Redeemable convertible preferred stock, to be issued					17,612,279
Series A Redeemable Convertible Preferred Stock [Member] | Sales Milestone [Member] | Maximum [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued and sold			13,333,333
Redeemable convertible preferred stock, sales price per share			$ 1.3125
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized						28,519,322
Redeemable convertible preferred stock, shares issued and sold	28,285,557
Redeemable convertible preferred stock, sales price per share	$ 2.1389
Proceeds from issuance of redeemable convertible preferred stock	$ 60,500
Redeemable convertible preferred stock, net of issuance costs	$ 2,243
Series B Redeemable Convertible Preferred Stock [Member] | BioMarin Pharmaceutical Inc [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares issued and sold	233,765