CONDENSED STATEMENTS OF REDEEMABLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Series B1 redeemable convertible preferred stock	Total
Beginning balance at Dec. 31, 2018	$ 27	$ 728,342	$ (664,375)			$ 63,994
Beginning balance (in shares) at Dec. 31, 2018	27,188
Sale of common stock, net of issuance costs	$ 1	1,584				1,585
Sale of common stock, net of issuance costs (in shares)	533
Issuance of commitment shares (Note 8) (in shares)	270
Issuance of common stock under employee stock purchase plan		26				26
Issuance of common stock under employee stock purchase plan (in shares)	11
Issuance of common stock for services rendered		23				23
Issuance of common stock for services rendered (in shares)	6
Stock-based compensation expense		1,016				1,016
Unrealized gain on marketable securities				$ 2		2
Net income (loss)			(10,974)			(10,974)
Ending balance at Mar. 31, 2019	$ 28	730,991	(675,349)	2		55,672
Ending balance (in shares) at Mar. 31, 2019	28,008
Beginning balance at Dec. 31, 2018	$ 27	728,342	(664,375)			63,994
Beginning balance (in shares) at Dec. 31, 2018	27,188
Net income (loss)						(22,150)
Ending balance at Jun. 30, 2019	$ 29	734,229	(686,525)	2		47,735
Ending balance (in shares) at Jun. 30, 2019	28,827
Beginning balance at Mar. 31, 2019	$ 28	730,991	(675,349)	2		55,672
Beginning balance (in shares) at Mar. 31, 2019	28,008
Sale of common stock, net of issuance costs	$ 1	2,271				2,272
Sale of common stock, net of issuance costs (in shares)	786
Issuance of common stock under employee stock purchase plan		42				42
Issuance of common stock under employee stock purchase plan (in shares)	19
Issuance of common stock for services rendered		36				36
Issuance of common stock for services rendered (in shares)	14
Stock-based compensation expense		889				889
Net income (loss)			(11,176)			(11,176)
Ending balance at Jun. 30, 2019	$ 29	734,229	(686,525)	$ 2		47,735
Ending balance (in shares) at Jun. 30, 2019	28,827
Beginning balance at Dec. 31, 2019	$ 30	709,692	(720,890)			$ (11,168)
Beginning balance (in shares) at Dec. 31, 2019	29,672				24	29,672
Sale of common stock, net of issuance costs	$ 1	1,405				$ 1,406
Sale of common stock, net of issuance costs (in shares)	854
Issuance of common stock under employee stock purchase plan		25				25
Issuance of common stock under employee stock purchase plan (in shares)	19
Issuance of common stock under equity incentive plan (vesting of restricted stock units)	48
Issuance of common stock for services rendered		26				26
Issuance of common stock for services rendered (in shares)	14
Stock-based compensation expense		750				750
Net income (loss)			8,817			8,817
Ending balance at Mar. 31, 2020	$ 31	711,898	(712,073)			(144)
Ending balance (in shares) at Mar. 31, 2020	30,607				24
Beginning balance at Dec. 31, 2019	$ 30	709,692	(720,890)			$ (11,168)
Beginning balance (in shares) at Dec. 31, 2019	29,672				24	29,672
Net income (loss)						$ (15,409)
Ending balance at Jun. 30, 2020	$ 34	718,574	(736,299)			$ (17,691)
Ending balance (in shares) at Jun. 30, 2020	34,291				24	34,291
Beginning balance at Mar. 31, 2020	$ 31	711,898	(712,073)			$ (144)
Beginning balance (in shares) at Mar. 31, 2020	30,607				24
Sale of common stock, net of issuance costs	$ 3	5,821				5,824
Sale of common stock, net of issuance costs (in shares)	3,607
Issuance of common stock under employee stock purchase plan		29				29
Issuance of common stock under employee stock purchase plan (in shares)	21
Issuance of common stock for services rendered		72				72
Issuance of common stock for services rendered (in shares)	56
Stock-based compensation expense		754				754
Net income (loss)			(24,226)			(24,226)
Ending balance at Jun. 30, 2020	$ 34	$ 718,574	$ (736,299)			$ (17,691)
Ending balance (in shares) at Jun. 30, 2020	34,291				24	34,291