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			98,000
Issuance of common stock to Roche, net of issuance costs	312,053			312,053
Issuance of common stock to Roche, net of issuance costs, shares			2,522,000
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, 2019	818,187		$ 8	3,112,130		50	(2,294,001)
Balance (in shares) at Dec. 31, 2019			75,185,000
Net loss	(364,811)
Balance at Sep. 30, 2020	892,047		$ 8	3,550,857		(6)	(2,658,812)
Balance (in shares) at Sep. 30, 2020			78,790,000
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
Exercise of options for common stock	21,860			21,860
Exercise of options for common stock, shares			449,000
Vest of restricted stock units			25,000
Stock-based compensation	27,616			27,616
Unrealized gains (losses) from available-for-sale securities, net of tax	(618)					618
Net loss	(150,820)						150,820
Balance at Jun. 30, 2020	1,042,866		$ 8	3,505,165		6	(2,462,313)
Balance (in shares) at Jun. 30, 2020			78,432,000
Exercise of options for common stock, shares			292,000
Vest of restricted stock units			20,000
Issuance of common stock under employee stock purchase plan	3,670			3,670
Issuance of common stock under employee stock purchase plan, shares			46,000
Stock-based compensation	26,903			26,903
Unrealized gains (losses) from available-for-sale securities, net of tax	(12)					12
Net loss	(196,499)						196,499
Balance at Sep. 30, 2020	892,047		$ 8	3,550,857		(6)	(2,658,812)
Balance (in shares) at Sep. 30, 2020			78,790,000
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
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
Net loss	(296,799)
Balance at Sep. 30, 2021	469,528		$ 8	3,554,307		(20)	(3,084,767)
Balance (in shares) at Sep. 30, 2021			79,959
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
Exercise of options for common stock	3,526			3,526
Exercise of options for common stock, shares			72,000
Vest of restricted stock units			28,000
Shares withheld for taxes	(1,432)			(1,432)
Shares withheld for taxes, shares			(18,000)
Stock-based compensation	28,969			28,969
Unrealized gains (losses) from available-for-sale securities, net of tax	5					5
Net loss	(81,405)						(81,405)
Balance at Jun. 30, 2021	485,108		$ 8	3,521,721		2	(3,036,623)
Balance (in shares) at Jun. 30, 2021			79,830,000
Exercise of options for common stock	2,606			2,606
Exercise of options for common stock, shares			52,000
Vest of restricted stock units			27,000
Issuance of common stock under employee stock purchase plan	3,296			3,296
Issuance of common stock under employee stock purchase plan, shares			50,000
Stock-based compensation	26,684			26,684
Unrealized gains (losses) from available-for-sale securities, net of tax	(22)					(22)
Net loss	(48,144)						(48,144)
Balance at Sep. 30, 2021	$ 469,528		$ 8	$ 3,554,307		$ (20)	$ (3,084,767)
Balance (in shares) at Sep. 30, 2021			79,959