Common Stock, Redeemable Common Stock, and Convertible Preferred Stock (converted to Common Stock) - Additional Information (Detail)		1 Months Ended	9 Months Ended
	Apr. 12, 2019 USD ($) $ / shares shares	Aug. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) vote $ / shares shares	Apr. 16, 2019 $ / shares	Dec. 31, 2018 $ / shares shares	Aug. 31, 2014 $ / shares
Class of Stock [Line Items]
Common stock, authorized shares | shares			33,333,333		11,070,776
Common stock, par value (in usd per share) | $ / shares			$ 0.001		$ 0.001
Number of votes per share | vote			1
Dividends on common stock declared or paid | $			$ 0
Issuance of preferred stock (in shares) | shares	5,670,000
Sale of stock, price per share (in usd per share) | $ / shares	$ 11.00
Proceeds from issuance common stock before underwriters discounts and commissions and offering expenses | $	$ 85,800,000
Convertible preferred stock, shares authorized (in shares) | shares					49,413,523
Preferred stock outstanding | $			$ 0
Series Seed Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares			10,000,000		59,413,523
Convertible preferred stock, par or stated value per share (in usd per share) | $ / shares			$ 0.001		$ 0.001
Genzyme Agreement
Class of Stock [Line Items]
Issuance of preferred stock (in shares) | shares		107,371
Sale of stock, price per share (in usd per share) | $ / shares						$ 0.01
Stock issued during period (in shares) | shares		107,371
Common stock aggregate fair value | $		$ 734,000
Common stock repurchase price per share (in usd per share) | $ / shares		$ 0.01
Pre Funded Warrant
Class of Stock [Line Items]
Common stock, par value (in usd per share) | $ / shares				$ 0.001
Class of warrant or right, warrants to purchase of common stock (in shares) | shares	2,130,000
Warrant exercise price (usd per share) | $ / shares	$ 10.999			10.999
Class A Warrant
Class of Stock [Line Items]
Class of warrant or right, warrants to purchase of common stock (in shares) | shares	3,900,000
Warrant exercise price (usd per share) | $ / shares	$ 13.20			$ 13.20
Class of warrants or rights expiration period (in years)	5 years