Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2016	$ 5	$ 453,044	$ (725)	$ (308,747)	$ 143,577
Balance (in shares) at Dec. 31, 2016	52,665,000
Issuance of common stock upon exercise of stock options	$ 1	2,314			2,315
Issuance of common stock upon exercise of stock options (in shares)	565,000
Issuance of common stock under employee stock purchase plan		3,344			3,344
Issuance of common stock under employee stock purchase plan (in shares)	446,000
Vesting of restricted stock		11,402			11,402
Vesting of restricted stock (in shares)	64,000
Unrealized gain (loss) on available-for-sale securities			(41)		(41)
Issuance of common stock in exchange for commitment to provide debt financing		2,448			2,448
Issuance of common stock in exchange for commitment to provide debt financing (in shares)	300,000
Net loss				(137,628)	(137,628)
Balance at Dec. 31, 2017	$ 6	472,552	(766)	(446,375)	25,417
Balance (in shares) at Dec. 31, 2017	54,040,000
Issuance of common stock upon exercise of stock options		13,331			13,331
Issuance of common stock upon exercise of stock options (in shares)	1,967,000
Issuance of common stock under employee stock purchase plan		3,617			3,617
Issuance of common stock under employee stock purchase plan (in shares)	391,000
Issuance of common stock upon exercise of warrants		6,762			6,762
Issuance of common stock upon exercise of warrants (in shares)	333,000
Sale of common stock through equity offering, net of issuance costs	$ 1	96,776			96,777
Sale of common stock through equity offering, net of issuance costs (in shares)	5,175,000
Vesting of restricted stock (in shares)	177,000
Stock based compensation		14,198			14,198
Unrealized gain (loss) on available-for-sale securities			214		214
Net loss				(128,154)	(128,154)
Balance at Dec. 31, 2018	$ 7	607,236	(552)	(574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083,000				62,083,000
Issuance of common stock upon exercise of stock options		13,041			$ 13,041
Issuance of common stock upon exercise of stock options (in shares)	2,464,000
Issuance of common stock under employee stock purchase plan		4,323			4,323
Issuance of common stock under employee stock purchase plan (in shares)	268,000
Sale of common stock through equity offering, net of issuance costs	$ 1	323,409			323,410
Sale of common stock through equity offering, net of issuance costs (in shares)	12,624,000
Issuance of common stock to Orbimed		507			507
Issuance of common stock to Orbimed (in shares)	25,000
Vesting of restricted stock (in shares)	541,000
Stock based compensation		28,624			28,624
Cumulative-effect adjustment upon adoption of ASU 2018-07		(185)		185
Unrealized gain (loss) on available-for-sale securities			1,471		1,471
Net loss				(124,827)	(124,827)
Balance at Dec. 31, 2019	$ 8	$ 976,955	$ 919	$ (699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005,000				78,004,669