CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Ionis [Member]	Common Stock [Member]	Common Stock [Member] Ionis [Member]	Additional Paid In Capital [Member]	Additional Paid In Capital [Member] Ionis [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Other Comprehensive Loss [Member] Ionis [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Ionis [Member]
Balance at Dec. 31, 2017	$ 167,825		$ 67		$ 464,430		$ (451)		$ (296,221)
Balance (in shares) at Dec. 31, 2017			66,542
Net income (loss)	(29,627)		$ 0		0		0		(29,627)
Change in unrealized gains (losses), net of tax	(45)		0		0		(45)		0
Currency translation adjustment	28		0		0		28		0
Exercise of common stock options	1,617		$ 0		1,617		0		0
Exercise of common stock options (in shares)			246
Issuance of common stock in connection with employee stock purchase plan	118		$ 0		118		0		0
Issuance of common stock in connection with employee stock purchase plan (in shares)			16
Stock-based compensation expense	6,384		$ 0		6,384		0		0
Balance at Mar. 31, 2018	146,300		$ 67		472,549		(468)		(325,848)
Balance (in shares) at Mar. 31, 2018			66,804
Balance at Dec. 31, 2018	276,724		$ 89		799,001		(324)		(522,042)
Balance (in shares) at Dec. 31, 2018			89,346
Net income (loss)	27,187		$ 0		0		0		27,187
Change in unrealized gains (losses), net of tax	212		0		0		212		0
Currency translation adjustment	85		0		0		85		0
Exercise of common stock options	4,206		$ 1		4,205		0		0
Exercise of common stock options (in shares)			434
Issuance of common stock in connection with employee stock purchase plan	382		$ 0		382		0		0
Issuance of common stock in connection with employee stock purchase plan (in shares)			18
Issuance of common stock in connection with Ionis sublicense fee		$ 75,000		$ 3		$ 74,997		$ 0		$ 0
Issuance of common stock in connection with Ionis sublicense fee (in shares)				2,837
Distribution to Ionis		$ (13,492)		$ 0		$ (13,492)		$ 0		$ 0
Stock-based compensation expense	18,560		$ 0		18,560		0		0
Balance at Mar. 31, 2019	$ 388,864		$ 93		$ 883,653		$ (27)		$ (494,855)
Balance (in shares) at Mar. 31, 2019			92,635