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 at Dec. 31, 2018			89,346
Net (loss) income	27,187							27,187
Change in unrealized gains (losses), net of tax	212						212
Currency translation adjustment	85						85
Issuance of common stock in connection with employee stock plans	4,588		$ 1		4,587
Issuance of common stock in connection with employee stock plans (in shares)			452
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	18,560				18,560
Balance at Mar. 31, 2019	388,864		$ 93		883,653		(27)	(494,855)
Balance at Mar. 31, 2019			92,635
Balance at Dec. 31, 2019	543,004		$ 101		1,024,168		5	(481,270)
Balance at Dec. 31, 2019			100,993
Net (loss) income	(42,850)							(42,850)
Change in unrealized gains (losses), net of tax	377						377
Currency translation adjustment	9						9
Issuance of common stock in connection with employee stock plans	1,945				1,945
Issuance of common stock in connection with employee stock plans (in shares)			360
Stock-based compensation expense	7,282				7,282
Payments of tax withholdings related to exercise of employee stock options	(76)				(76)
Payments of tax withholdings related to vesting of employee stock options (in shares)			(4)
Balance at Mar. 31, 2020	$ 509,691		$ 101		$ 1,033,319		$ 391	$ (524,120)
Balance at Mar. 31, 2020			101,349