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		4,196			4,196
Issuance of common stock upon exercise of stock options (in shares)	788
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
Vesting of restricted stock (in shares)	87
Stock based compensation		6,522			6,522
Unrealized gain (loss) on available-for-sale securities			(92)		(92)
Net loss				(66,697)	(66,697)
Balance at Jun. 30, 2018	$ 6	491,887	(858)	(513,072)	(22,037)
Balance (in shares) at Jun. 30, 2018	55,454
Balance at Mar. 31, 2018	$ 6	476,270	(904)	(479,248)	(3,876)
Balance (in shares) at Mar. 31, 2018	54,251
Issuance of common stock upon exercise of stock options		3,632			3,632
Issuance of common stock upon exercise of stock options (in shares)	643
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
Vesting of restricted stock (in shares)	21
Stock based compensation		3,368			3,368
Unrealized gain (loss) on available-for-sale securities			46		46
Net loss				(33,824)	(33,824)
Balance at Jun. 30, 2018	$ 6	491,887	(858)	(513,072)	(22,037)
Balance (in shares) at Jun. 30, 2018	55,454
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	$ 93,088
Balance (in shares) at Jun. 30, 2019	70,035				70,035
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		5,983			$ 5,983
Issuance of common stock upon exercise of stock options (in shares)	1,468
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 public offering, net		107,595			107,595
Issuance of common stock for public 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)	274
Stock based compensation		9,855			9,855
Unrealized gain (loss) on available-for-sale securities			1,347		1,347
Net loss				(66,507)	(66,507)
Balance at Jun. 30, 2019	$ 7	733,137	795	(640,851)	$ 93,088
Balance (in shares) at Jun. 30, 2019	70,035				70,035
Balance at Mar. 31, 2019	$ 7	613,680	(266)	(608,435)	$ 4,986
Balance (in shares) at Mar. 31, 2019	63,340
Issuance of common stock upon exercise of stock options		3,405			3,405
Issuance of common stock upon exercise of stock options (in shares)	398
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 public offering, net		107,595			107,595
Issuance of common stock for public 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)	87
Stock based compensation		5,804			5,804
Unrealized gain (loss) on available-for-sale securities			1,061		1,061
Net loss				(32,416)	(32,416)
Balance at Jun. 30, 2019	$ 7	$ 733,137	$ 795	$ (640,851)	$ 93,088
Balance (in shares) at Jun. 30, 2019	70,035				70,035