Condensed Consolidated Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2018	$ (48,045)	$ 41	$ 341,284	$ (389,370)
Beginning balance, shares at Dec. 31, 2018		40,936,735
Issuance of common stock upon exercise of stock options, value	120		120
Issuance of common stock upon exercise of stock options, shares		38,125
Issuance of common stock upon vesting of RSUs, net of tax withholdings	153		153
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		73,500
Issuance of common stock under ESPP plan, value	207		207
Issuance of common stock under ESPP plan, shares		46,472
Stock-based compensation expense	2,065		2,065
Net loss	(24,333)			(24,333)
Ending balance at Mar. 31, 2019	(69,833)	$ 41	343,829	(413,703)
Ending balance, shares at Mar. 31, 2019		41,094,832
Beginning balance at Dec. 31, 2018	(48,045)	$ 41	341,284	(389,370)
Beginning balance, shares at Dec. 31, 2018		40,936,735
Net loss	(35,092)
Ending balance at Jun. 30, 2019	(17,968)	$ 70	406,424	(424,462)
Ending balance, shares at Jun. 30, 2019		69,913,410
Beginning balance at Mar. 31, 2019	(69,833)	$ 41	343,829	(413,703)
Beginning balance, shares at Mar. 31, 2019		41,094,832
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, value	60,527	$ 29	60,498
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares		28,818,578
Stock-based compensation expense	2,097		2,097
Net loss	(10,759)			(10,759)
Ending balance at Jun. 30, 2019	(17,968)	$ 70	406,424	(424,462)
Ending balance, shares at Jun. 30, 2019		69,913,410
Beginning balance at Dec. 31, 2019	$ (48,324)	$ 70	411,255	(459,649)
Beginning balance, shares at Dec. 31, 2019	70,143,252	70,143,252
Issuance of common stock from at the market equity offering	$ 4,178	$ 1	4,177
Issuance of common stock from at the market equity offering, shares		1,230,531
Issuance of common stock upon exercise of stock options, value	60	$ 1	59
Issuance of common stock upon exercise of stock options, shares		110,967
Issuance of common stock upon vesting of RSUs, net of tax withholdings	120		120
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		110,000
Issuance of common stock under ESPP plan, value	249		249
Issuance of common stock under ESPP plan, shares		76,317
Stock-based compensation expense	1,959		1,959
Unrealized gain (loss) on investments	(10)				$ (10)
Net loss	(19,881)			(19,881)
Ending balance at Mar. 31, 2020	(61,649)	$ 72	417,819	(479,530)	(10)
Ending balance, shares at Mar. 31, 2020		71,671,067
Beginning balance at Dec. 31, 2019	$ (48,324)	$ 70	411,255	(459,649)
Beginning balance, shares at Dec. 31, 2019	70,143,252	70,143,252
Issuance of common stock upon exercise of stock options, shares	141,281
Net loss	$ (40,595)
Ending balance at Jun. 30, 2020	$ (65,575)	$ 75	434,593	(500,244)	1
Ending balance, shares at Jun. 30, 2020	75,131,834	75,131,834
Beginning balance at Mar. 31, 2020	$ (61,649)	$ 72	417,819	(479,530)	(10)
Beginning balance, shares at Mar. 31, 2020		71,671,067
Issuance of common stock from at the market equity offering	14,848	$ 3	14,845
Issuance of common stock from at the market equity offering, shares		3,430,453
Issuance of common stock upon exercise of stock options, value	15		15
Issuance of common stock upon exercise of stock options, shares		30,314
Stock-based compensation expense	1,914		1,914
Unrealized gain (loss) on investments	11				11
Net loss	(20,714)			(20,714)
Ending balance at Jun. 30, 2020	$ (65,575)	$ 75	$ 434,593	$ (500,244)	$ 1
Ending balance, shares at Jun. 30, 2020	75,131,834	75,131,834