Initial Public Offering (Details) $ in Millions	Jul. 19, 2017 USD ($) shares
Initial Public Offering [Abstract]
Net proceeds from sale of common stock in IPO	$ 182.3
Gross proceeds from sale of common stock in IPO	132.3
Investment by Ionis	25.0
Proceeds from sale of common stock to Novartis in a private placement	$ 50.0
Shares issued upon conversion of Series A Convertible Preferred Stock (in shares) | shares	28,884,540
Principal and accrued interest from line of credit converted	$ 106.0
Shares issued upon conversion of line of credit (in shares) | shares	13,438,339
IPO [Member]
Initial Public Offering [Abstract]
Shares issued (in shares) | shares	17,968,750
Private Placement [Member]
Initial Public Offering [Abstract]
Shares issued (in shares) | shares	6,250,000