Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ 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		71,072,000
Exercise of options for common stock	9,973		9,973
Exercise of options for common stock, shares		382,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		35,000
Shares withheld for taxes	$ (889)		$ (889)
Shares withheld for taxes, shares		(7,000)
Issuance of common stock to Roche, net of issuance costs	365,264	2,604,000	365,264
Issuance of common stock under employee stock purchase plan	$ 2,326		$ 2,326
Issuance of common stock under employee stock purchase plan, shares		48,000
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		74,134,000
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
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	2,522,000	312,053
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 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