CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Ionis [Member] TTR License Agreement [Member]	Ionis [Member] TEGSEDI [Member]	Common Stock [Member]	Common Stock [Member] Ionis [Member] TTR License Agreement [Member]	Common Stock [Member] Ionis [Member] TEGSEDI [Member]	Additional Paid In Capital [Member]	Additional Paid In Capital [Member] Ionis [Member] TTR License Agreement [Member]	Additional Paid In Capital [Member] Ionis [Member] TEGSEDI [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 167,825			$ 67			$ 464,430			$ (451)	$ (296,221)
Balance (in shares) at Dec. 31, 2017				66,542
Net loss	(155,264)										(155,264)
Change in unrealized gains, net of tax	143									143
Currency translation adjustment	19									19
Exercise of common stock options	4,800						4,800
Exercise of common stock options (in shares)				633
Issuance of stock		$ 200,100	$ 40,000		$ 19	$ 1		$ 200,081	$ 39,999
Issuance of stock (in shares)					18,667	1,597
Distribution in connection with license transaction		(7,792)						(7,792)
Issuance of common stock in connection with employee stock purchase plan	341						341
Issuance of common stock in connection with employee stock purchase plan (in shares)				32
Stock-based compensation expense	31,239						31,239
Balance at Sep. 30, 2018	281,411			$ 87			733,098			(289)	(451,485)
Balance (in shares) at Sep. 30, 2018				87,471
Balance at Jun. 30, 2018	290,404			$ 86			678,509			(297)	(387,894)
Balance (in shares) at Jun. 30, 2018				85,681
Net loss	(63,591)										(63,591)
Change in unrealized gains, net of tax	37									37
Currency translation adjustment	(29)									(29)
Exercise of common stock options	1,626						1,626
Exercise of common stock options (in shares)				177
Issuance of stock			$ 40,000			$ 1			$ 39,999
Issuance of stock (in shares)						1,597
Issuance of common stock in connection with employee stock purchase plan	234						234
Issuance of common stock in connection with employee stock purchase plan (in shares)				16
Stock-based compensation expense	12,730						12,730
Balance at Sep. 30, 2018	281,411			$ 87			733,098			(289)	(451,485)
Balance (in shares) at Sep. 30, 2018				87,471
Balance at Dec. 31, 2018	276,724			$ 89			799,001			(324)	(522,042)
Balance (in shares) at Dec. 31, 2018				89,346
Net loss	(41,634)										(41,634)
Change in unrealized gains, net of tax	286									286
Currency translation adjustment	(25)									(25)
Exercise of common stock options	8,363			$ 1			8,362
Exercise of common stock options (in shares)				2,264
Issuance of stock		75,000			$ 3			74,997
Issuance of stock (in shares)					2,837
Distribution in connection with license transaction		$ (13,492)						$ (13,492)
Issuance of common stock in connection with employee stock purchase plan	822						822
Issuance of common stock in connection with employee stock purchase plan (in shares)				40
Stock-based compensation expense	29,459						29,459
Payments of tax withholdings related to exercise of employee stock awards	(10,015)			$ 1			(10,016)
Payments of tax withholdings related to vesting of employee stock awards (in shares)				(602)
Balance at Sep. 30, 2019	325,488			$ 94			889,133			(63)	(563,676)
Balance (in shares) at Sep. 30, 2019				93,885
Balance at Jun. 30, 2019	368,642			$ 93			900,837			(110)	(532,178)
Balance (in shares) at Jun. 30, 2019				92,921
Net loss	(31,498)										(31,498)
Change in unrealized gains, net of tax	64									64
Currency translation adjustment	(17)									(17)
Exercise of common stock options	1,335						1,335
Exercise of common stock options (in shares)				1,544
Issuance of common stock in connection with employee stock purchase plan	441						441
Issuance of common stock in connection with employee stock purchase plan (in shares)				22
Stock-based compensation expense	(3,464)						(3,464)
Payments of tax withholdings related to exercise of employee stock awards	(10,015)			$ 1			(10,016)
Payments of tax withholdings related to vesting of employee stock awards (in shares)				(602)
Balance at Sep. 30, 2019	$ 325,488			$ 94			$ 889,133			$ (63)	$ (563,676)
Balance (in shares) at Sep. 30, 2019				93,885