Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect of Accounting Change [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Cumulative Effect of Accounting Change [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Cumulative Effect of Accounting Change [Member]
Balance at Dec. 31, 2019	$ 818,187		$ 8	$ 3,112,130		$ 50	$ (2,294,001)
Balance (in shares) at Dec. 31, 2019			75,185,000
Exercise of options for common stock	3,687			3,687
Exercise of options for common stock, shares			97,000
Vest of restricted stock units			98
Issuance of common stock to Roche, net ofissuance costs	312,053			312,053
Issuance of common stock to Roche, net of issuance costs, shares			2,522
Issuance of common stock under employee stock purchase plan	3,795			3,795
Issuance of common stock under employee stock purchase plan, shares			56,000
Stock-based compensation	24,024			24,024
Unrealized gains (losses) from available-for-sale securities, net of tax	574					574
Net loss	(17,492)						(17,492)
Balance at Mar. 31, 2020	1,144,828		$ 8	3,455,689		624	(2,311,493)
Balance (in shares) at Mar. 31, 2020			77,958,000
Balance at Dec. 31, 2020	761,759	$ (96,792)	$ 8	3,609,877	$ (156,953)	3	(2,848,129)	$ 60,161
Balance (in shares) at Dec. 31, 2020			79,374,000
Exercise of options for common stock	4,683			4,683
Exercise of options for common stock, shares			108,000
Vest of restricted stock units			204,000
Issuance of common stock under employee stock purchase plan	4,543			4,543
Issuance of common stock under employee stock purchase plan, shares			62,000
Stock-based compensation	28,508			28,508
Unrealized gains (losses) from available-for-sale securities, net of tax	(6)					(6)
Net loss	(167,250)						(167,250)
Balance at Mar. 31, 2021	$ 535,445		$ 8	$ 3,490,658		$ (3)	$ (2,955,218)
Balance (in shares) at Mar. 31, 2021			79,748,000