Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Net loss	$ (49,577)	$ (37,323)	$ (92,427)	$ (10,136)
Distributions to Ionis				(13,492)
Distributable losses	$ (49,577)	$ (37,323)	$ (92,427)	$ (23,628)
Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Weighted-average common shares outstanding owned	101,476,274	92,795,238	101,290,672	91,757,549
Ionis [Member] | Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Net loss	$ (37,665)	$ (28,244)	$ (70,348)	$ (4,380)
Distributions to Ionis				13,492
Distributable losses	$ (37,665)	$ (28,244)	$ (70,348)	$ (17,872)
Weighted-average common shares outstanding owned	77,094,682	70,221,338	77,094,682	69,406,181
Basic (loss) per common share	$ (0.49)	$ (0.40)	$ (0.91)	$ (0.06)
Others [Member] | Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Distributable losses	$ (11,912)	$ (9,079)	$ (22,079)	$ (5,756)
Weighted-average common shares outstanding owned	24,381,592	22,573,900	24,195,990	22,351,368
Basic (loss) per common share	$ (0.49)	$ (0.40)	$ (0.91)	$ (0.26)