Subsequent Events (Details) - USD ($) $ in Thousands	Jul. 19, 2017	Jun. 30, 2017	Dec. 31, 2016
Balance Sheet Data [Abstract]
Cash and cash equivalents and short-term investments		$ 118,668
Other current assets		3,939	$ 1,209
Accounts payable and accrued expenses		3,608
Line of credit with Ionis Pharmaceuticals, Inc.		107,507	0
Convertible preferred stock		100,000	100,000
Common stock		0	0
Additional paid-in capital		64,059	56,936
Total stockholders' equity (deficit)		(82,983)	$ (17,747)
Pro Forma [Member]
Balance Sheet Data [Abstract]
Cash and cash equivalents and short-term investments		302,382
Other current assets		2,052
Accounts payable and accrued expenses		2,998
Line of credit with Ionis Pharmaceuticals, Inc.		0
Convertible preferred stock		0
Common stock		67
Additional paid-in capital		453,936
Total stockholders' equity (deficit)		$ 206,961
Subsequent Event [Member]
Subsequent Events [Abstract]
Net proceeds from sale of common stock	$ 182,400
Shares issued upon conversion of Series A Convertible Preferred Stock (in shares)	28,884,540
Principal and accrued interest from line of credit converted	$ 106,000
Shares issued upon conversion of line of credit (in shares)	13,438,339
Subsequent Event [Member] | IPO [Member]
Subsequent Events [Abstract]
Gross proceeds from sale of common stock in IPO	$ 132,400
Shares issued (in shares)	17,968,750
Subsequent Event [Member] | Private Placement [Member] | Novartis [Member]
Subsequent Events [Abstract]
Gross proceeds from sale of common stock in IPO	$ 50,000
Shares issued (in shares)	6,250,000