Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2018	$ 1,032,276	$ 7	$ 2,611,294	$ (99)	$ (1,578,926)
Balance (in shares) at Dec. 31, 2018		71,072
Exercise of options for common stock	9,973		9,973
Exercise of options for common stock, shares		382
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		35
Shares withheld for taxes	(889)		(889)
Shares withheld for taxes, shares		(7)
Issuance of common stock to Roche, net of issuance costs	365,264		365,264
Issuance of common stock to Roche, net of issuance costs, shares		2,604
Issuance of common stock under employee stock purchase plan	2,326		2,326
Issuance of common stock under employee stock purchase plan, shares		48
Stock-based compensation	16,139		16,139
Unrealized gains from available-for-sale securities, net of tax	118			118
Net loss	(76,643)				(76,643)
Balance at Mar. 31, 2019	1,348,564	$ 7	3,004,107	19	(1,655,569)
Balance (in shares) at Mar. 31, 2019		74,134
Balance at Dec. 31, 2018	1,032,276	$ 7	2,611,294	(99)	(1,578,926)
Balance (in shares) at Dec. 31, 2018		71,072
Net loss	(353,046)
Balance at Jun. 30, 2019	1,099,167	$ 7	3,031,050	82	(1,931,972)
Balance (in shares) at Jun. 30, 2019		74,328
Balance at Mar. 31, 2019	1,348,564	$ 7	3,004,107	19	(1,655,569)
Balance (in shares) at Mar. 31, 2019		74,134
Exercise of options for common stock	7,092		7,092
Exercise of options for common stock, shares		176
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		18
Issuance of common stock to Roche, net of issuance costs	89		89
Stock-based compensation	19,762		19,762
Unrealized gains from available-for-sale securities, net of tax	63			63
Net loss	(276,403)				(276,403)
Balance at Jun. 30, 2019	1,099,167	$ 7	3,031,050	82	(1,931,972)
Balance (in shares) at Jun. 30, 2019		74,328
Balance at Dec. 31, 2019	818,187	$ 8	3,112,130	50	(2,294,001)
Balance (in shares) at Dec. 31, 2019		75,185
Exercise of options for common stock	3,687		3,687
Exercise of options for common stock, shares		97
Vest of restricted stock units		98
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
Issuance of common stock under employee stock purchase plan	3,795		3,795
Issuance of common stock under employee stock purchase plan, shares		56
Stock-based compensation	24,024		24,024
Unrealized gains 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
Balance at Dec. 31, 2019	818,187	$ 8	3,112,130	50	(2,294,001)
Balance (in shares) at Dec. 31, 2019		75,185
Net loss	(168,312)
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
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
Exercise of options for common stock	21,860		21,860
Exercise of options for common stock, shares		449
Vest of restricted stock units		25
Stock-based compensation	27,616		27,616
Unrealized gains 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