Preferred and Common Stock (Details)						1 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Apr. 03, 2018 shares	Oct. 10, 2017 USD ($) $ / shares shares	Sep. 22, 2017	Aug. 21, 2017 shares	Oct. 31, 2017	Aug. 31, 2017 shares	Sep. 30, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares	Mar. 31, 2013 $ / shares shares
Preferred Stock
Temporary equity, par value per share | $ / shares								$ 0.001	$ 0.001
Net proceeds | $	$ 162,899
Common stock, issued								34,382,525	27,284,140	10,196,292
Purchase price per share | $ / shares								$ 0.001	$ 0.001	$ 0.001
Stock split				0.109051
IPO
Preferred Stock
Issuance of stock (in shares)	6,591,800
Minimum price per share | $ / shares	$ 26.42
Number of shares issued on conversion							9.17
Underwriter option to purchase
Preferred Stock
Issuance of stock (in shares)	859,800
Gross proceeds | $	$ 174,155
Common stock
Preferred Stock
Issuance of common stock on exchange of preferred stock					8,578,646
Issuance of common stock on exchange of common stock units					1,617,646
Common stock | IPO
Preferred Stock
Issuance of stock (in shares)			8,107,500
Minimum price per share | $ / shares			$ 17.00
Gross proceeds | $			$ 137,828,000
Net proceeds | $			$ 125,658,000
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock (in shares)			17,406,338
Common stock | Underwriter option to purchase
Preferred Stock
Issuance of stock (in shares)			1,057,500
Series A‑1 Junior Preferred Stock
Preferred Stock
Issuance of common stock on exchange of preferred stock					78,666,209
Conversion ratio						9.17
License agreement | Takeda
Preferred Stock
Issuance of stock (in shares)		223,544