Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2017	$ 6	$ 472,552	$ (766)	$ (446,375)	$ 25,417
Balance (in shares) at Dec. 31, 2017	54,040
Issuance of common stock upon exercise of stock options		12,063			12,063
Issuance of common stock upon exercise of stock options (in shares)	1,773
Issuance of common stock under employee stock purchase plan		1,855			1,855
Issuance of common stock under employee stock purchase plan (in shares)	206
Issuance of common stock upon exercise of warrants		6,762			6,762
Issuance of common stock upon exercise of warrants (in shares)	333
Issuance of common stock for offering, net	$ 1	96,776			96,777
Issuance of common stock for offering, net (in shares)	5,175
Vesting of restricted stock (in shares)	146
Stock based compensation		10,138			10,138
Unrealized gain on available-for-sale securities			(128)		(128)
Net loss				(96,313)	(96,313)
Balance at Sep. 30, 2018	$ 7	600,146	(894)	(542,688)	56,571
Balance (in shares) at Sep. 30, 2018	61,673
Balance at Jun. 30, 2018	$ 6	491,887	(858)	(513,072)	(22,037)
Balance (in shares) at Jun. 30, 2018	55,454
Issuance of common stock upon exercise of stock options		7,867			7,867
Issuance of common stock upon exercise of stock options (in shares)	985
Issuance of common stock for offering, net	$ 1	96,776			96,777
Issuance of common stock for offering, net (in shares)	5,175
Vesting of restricted stock (in shares)	59
Stock based compensation		3,616			3,616
Unrealized gain on available-for-sale securities			(36)		(36)
Net loss				(29,616)	(29,616)
Balance at Sep. 30, 2018	$ 7	600,146	(894)	(542,688)	56,571
Balance (in shares) at Sep. 30, 2018	61,673
Balance at Dec. 31, 2018	$ 7	607,236	(552)	(574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083				62,083
Cumulative-effect adjustment upon adoption of ASU 2018-07		(185)		185
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
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
Issuance of common stock for offering, net		107,595			107,595
Issuance of common stock for offering, net (in shares)	6,053
Issuance of common stock to Orbimed		506			506
Issuance of common stock to Orbimed (in shares)	25
Vesting of restricted stock (in shares)	419
Stock based compensation		20,102			20,102
Unrealized gain on available-for-sale securities			1,410		1,410
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				70,850
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	$ 93,088
Balance (in shares) at Jun. 30, 2019	70,035
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
Vesting of restricted stock (in shares)	145
Stock based compensation		10,247			10,247
Unrealized gain on available-for-sale securities			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				70,850