Basic and Diluted Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Net (loss) income	$ (42,850)	$ 27,187
Distributions to Ionis		(13,492)
Distributable (losses) income	$ (42,850)	$ 13,695
Weighted-average shares outstanding owned		94,127,942
Shares issuable related to our ESPP		33,982
Weighted-average shares outstanding owned by others, plus assumed conversions		25,545,975
Stock Options [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Weighted-average shares issuable		3,370,817
Restricted Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Weighted-average shares issuable		14,813
Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Weighted-average common shares outstanding owned	101,105,070	90,708,330
Ionis [Member] | Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Net (loss) income	$ (32,674)	$ 23,846
Distributions to Ionis		13,492
Distributable (losses) income	$ (32,674)	$ 10,354
Weighted-average common shares outstanding owned	77,094,682	68,581,967
Basic (loss) income per common share	$ (0.42)	$ 0.35
Weighted-average shares outstanding owned	77,094,682	68,581,967
Income allocated to Ionis		$ 9,978
Income available to Ionis		$ 23,470
Diluted income per common share	$ (0.42)	$ 0.34
Others [Member] | Common Stock [Member]
Basic and Diluted Net (Loss) Income Per Share [Abstract]
Distributable (losses) income	$ (10,176)	$ 3,341
Weighted-average common shares outstanding owned	24,010,388	22,126,363
Basic (loss) income per common share	$ (0.42)	$ 0.15
Weighted-average shares outstanding owned	24,010,388	25,545,975
Weighted-average shares outstanding owned by others, plus assumed conversions		25,545,975
Income allocated to Ionis		$ 3,717
Diluted income per common share	$ (0.42)	$ 0.15