CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Ionis [Member] TTR License Agreement [Member]	Common Stock [Member]	Common Stock [Member] Ionis [Member] TTR License Agreement [Member]	Additional Paid In Capital [Member]	Additional Paid In Capital [Member] Ionis [Member] TTR License Agreement [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2018	$ 276,724		$ 89		$ 799,001		$ (324)	$ (522,042)
Balance (in shares) at Dec. 31, 2018			89,346
Net loss	(10,136)							(10,136)
Change in unrealized gains, net of tax	222						222
Currency translation adjustment	(8)						(8)
Issuance of common stock in connection with employee stock plans	7,409		$ 1		7,408
Issuance of common stock in connection with employee stock plans (in shares)			738
Issuance of common stock in connection with Ionis sublicense fee		$ 75,000		$ 3		$ 74,997
Issuance of stock (in shares)				2,837
Distribution to Ionis		$ (13,492)				$ (13,492)
Stock-based compensation expense	32,923				32,923
Balance at Jun. 30, 2019	368,642		$ 93		900,837		(110)	(532,178)
Balance (in shares) at Jun. 30, 2019			92,921
Balance at Mar. 31, 2019	388,864		$ 93		883,653		(27)	(494,855)
Balance (in shares) at Mar. 31, 2019			92,635
Net loss	(37,323)							(37,323)
Change in unrealized gains, net of tax	10						10
Currency translation adjustment	(93)						(93)
Issuance of common stock in connection with employee stock plans	2,821				2,821
Issuance of common stock in connection with employee stock plans (in shares)			286
Stock-based compensation expense	14,363				14,363
Balance at Jun. 30, 2019	368,642		$ 93		900,837		(110)	(532,178)
Balance (in shares) at Jun. 30, 2019			92,921
Balance at Dec. 31, 2019	543,004		$ 101		1,024,168		5	(481,270)
Balance (in shares) at Dec. 31, 2019			100,993
Net loss	(92,427)							(92,427)
Change in unrealized gains, net of tax	1,393						1,393
Currency translation adjustment	83						83
Issuance of common stock in connection with employee stock plans	3,649		$ 1		3,648
Issuance of common stock in connection with employee stock plans (in shares)			520
Stock-based compensation expense	23,402				23,402
Payments of tax withholdings related to exercise of employee stock options/awards	(76)		$ (4)		(76)
Balance at Jun. 30, 2020	479,028		$ 102		1,051,142		1,481	(573,697)
Balance (in shares) at Jun. 30, 2020			101,509
Payments of tax withholdings related to exercise of employee stock options	76		$ 4		76
Balance at Mar. 31, 2020	509,691		$ 101		1,033,319		391	(524,120)
Balance (in shares) at Mar. 31, 2020			101,349
Net loss	(49,577)							(49,577)
Change in unrealized gains, net of tax	1,016						1,016
Currency translation adjustment	74						74
Issuance of common stock in connection with employee stock plans	1,703		$ 1		1,702
Issuance of common stock in connection with employee stock plans (in shares)			160
Stock-based compensation expense	16,121				16,121
Balance at Jun. 30, 2020	$ 479,028		$ 102		$ 1,051,142		$ 1,481	$ (573,697)
Balance (in shares) at Jun. 30, 2020			101,509