Basic and Diluted Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic Net Income (Loss) Per Share [Abstract]
Net income (loss)	$ 27,187	$ (29,627)
Distributions to Ionis	(13,492)	0
Distributable income (losses)	$ 13,695	$ (29,627)
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 Net Income (Loss) Per Share [Abstract]
Weighted-average shares issuable	3,370,817
Restricted Stock [Member]
Basic Net Income (Loss) Per Share [Abstract]
Weighted-average shares issuable	14,813
Common Stock [Member]
Basic Net Income (Loss) Per Share [Abstract]
Weighted-average shares outstanding, basic (in shares)	90,708,330	66,619,251
Weighted-average shares outstanding, diluted (in shares)	94,127,942
Common Stock [Member] | Ionis [Member]
Basic Net Income (Loss) Per Share [Abstract]
Net income (loss)	$ 23,846	$ (20,212)
Distributable income (losses)	$ 10,354	$ (20,212)
Weighted-average shares outstanding, basic (in shares)	68,581,967	45,447,879
Net income (loss) per share, basic (in dollars per share)	$ 0.35	$ (0.44)
Weighted-average shares outstanding, diluted (in shares)	68,581,967	45,447,879
Income allocated to Ionis	$ 9,978
Income available to Ionis	$ 23,470
Net income (loss) per share, diluted (in dollars per share)	$ 0.34	$ (0.44)
Common Stock [Member] | Others [Member]
Basic Net Income (Loss) Per Share [Abstract]
Distributable income (losses)	$ 3,341	$ (9,415)
Weighted-average shares outstanding, basic (in shares)	22,126,363	21,171,372
Net income (loss) per share, basic (in dollars per share)	$ 0.15	$ (0.44)
Weighted-average shares outstanding, diluted (in shares)	25,545,975	21,171,372
Weighted-average shares outstanding owned by others, plus assumed conversions	25,545,975
Income allocated to Ionis	$ 3,717
Net income (loss) per share, diluted (in dollars per share)	$ 0.15	$ (0.44)