Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 7	$ 607,236	$ (552)	$ (574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083,000
Issuance of common stock upon exercise of stock options		10,416			10,416
Issuance of common stock upon exercise of stock options (in shares)	2,138,000
Issuance of common stock under employee stock purchase plan		2,147			2,147
Issuance of common stock under employee stock purchase plan (in shares)	132,000
Issuance of common stock for public offering, net		107,595			107,595
Issuance of common stock for public offering, net (in shares)	6,053,000
Issuance of common stock to Orbimed		506			506
Issuance of common stock to Orbimed (in shares)	25,000
Vesting of restricted stock (in shares)	419,000
Stock based compensation		20,102			20,102
Unrealized gain (loss) on available-for-sale securities/ ST investments			1,410		1,410
Cumulative-effect adjustment upon adoption		(185)		185
Net loss				(89,644)	(89,644)
Balance at Sep. 30, 2019	$ 7	747,817	858	(663,988)	84,694
Balance (in shares) at Sep. 30, 2019	70,850,000
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	93,088
Balance (in shares) at Jun. 30, 2019	70,035,000
Issuance of common stock upon exercise of stock options		4,433			4,433
Issuance of common stock upon exercise of stock options (in shares)	670,000
Vesting of restricted stock (in shares)	145,000
Stock based compensation		10,247			10,247
Unrealized gain (loss) on available-for-sale securities/ ST investments			63		63
Net loss				(23,137)	(23,137)
Balance at Sep. 30, 2019	$ 7	747,817	858	(663,988)	84,694
Balance (in shares) at Sep. 30, 2019	70,850,000
Balance at Dec. 31, 2019	$ 8	976,955	919	(699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005,000				78,005,000
Issuance of common stock upon exercise of stock options		15,782			$ 15,782
Issuance of common stock upon exercise of stock options (in shares)	1,534,000
Issuance of common stock under employee stock purchase plan		3,062			3,062
Issuance of common stock under employee stock purchase plan (in shares)	97,000
Equity component of Convertible Notes, net		82,873			82,873
Issuance of common stock for public offering, net	$ 1	270,649			$ 270,650
Issuance of common stock for public offering, net (in shares)	4,792,000
Issuance of common stock to Orbimed (in shares)					25,000
Vesting of restricted stock (in shares)	853,000
Stock based compensation		34,406			$ 34,406
Unrealized gain (loss) on available-for-sale securities/ ST investments			4,251		4,251
Cumulative-effect adjustment upon adoption				(404)	(404)
Net loss				(153,331)	(153,331)
Balance at Sep. 30, 2020	$ 9	1,383,727	5,170	(852,906)	$ 536,000
Balance (in shares) at Sep. 30, 2020	85,281,000				85,281,000
Balance at Jun. 30, 2020	$ 8	1,093,072	5,770	(794,583)	$ 304,267
Balance (in shares) at Jun. 30, 2020	79,717,000
Issuance of common stock upon exercise of stock options		4,974			4,974
Issuance of common stock upon exercise of stock options (in shares)	498,000
Issuance of common stock for public offering, net	$ 1	270,649			270,650
Issuance of common stock for public offering, net (in shares)	4,792,000
Vesting of restricted stock (in shares)	274,000
Stock based compensation		15,032			15,032
Unrealized gain (loss) on available-for-sale securities/ ST investments			(600)		(600)
Net loss				(58,323)	(58,323)
Balance at Sep. 30, 2020	$ 9	$ 1,383,727	$ 5,170	$ (852,906)	$ 536,000
Balance (in shares) at Sep. 30, 2020	85,281,000				85,281,000