Condensed Statement of Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Common Stock and Common Warrants [Member]	Pre-funded Warrants and Common Warrants [Member]	Common Stock [Member]	Common Stock [Member] Common Stock and Common Warrants [Member]	Common Stock [Member] Pre-funded Warrants and Common Warrants [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Common Stock and Common Warrants [Member]	Additional Paid-in Capital [Member] Pre-funded Warrants and Common Warrants [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 47,797			$ 15			$ 184,761			$ (33)	$ (136,946)
Beginning balance, shares at Dec. 31, 2017				14,723,818
Exercise of stock options	16						16
Exercise of stock options, shares				10,565
Stock-based compensation expense	767						767
Unrealized gain (loss) on investments	(17)									(17)
Net loss	(7,588)										(7,588)
Ending balance at Mar. 31, 2018	40,975			$ 15			185,544			(50)	(144,534)
Ending balance, shares at Mar. 31, 2018				14,734,383
Beginning balance at Dec. 31, 2017	47,797			$ 15			184,761			(33)	(136,946)
Beginning balance, shares at Dec. 31, 2017				14,723,818
Unrealized gain (loss) on investments	21
Net loss	(17,079)
Ending balance at Jun. 30, 2018	32,868			$ 15			186,890			(12)	(154,025)
Ending balance, shares at Jun. 30, 2018				14,737,402
Beginning balance at Mar. 31, 2018	40,975			$ 15			185,544			(50)	(144,534)
Beginning balance, shares at Mar. 31, 2018				14,734,383
Exercise of stock options	12						12
Exercise of stock options, shares				3,019
Stock-based compensation expense	1,334						1,334
Unrealized gain (loss) on investments	38									38
Net loss	(9,491)										(9,491)
Ending balance at Jun. 30, 2018	32,868			$ 15			186,890			(12)	(154,025)
Ending balance, shares at Jun. 30, 2018				14,737,402
Beginning balance at Dec. 31, 2018	$ 19,600			$ 15			188,083			(5)	(168,493)
Beginning balance, shares at Dec. 31, 2018	14,748,475			14,748,475
Exercise of stock options	$ 165						165
Exercise of stock options, shares				126,560
Stock-based compensation expense	563						563
Adoption of ASC 842, Leases | ASU No. 2016-02 [Member]	(273)										(273)
Unrealized gain (loss) on investments	5									5
Net loss	(7,213)										(7,213)
Ending balance at Mar. 31, 2019	12,847			$ 15			188,811				(175,979)
Ending balance, shares at Mar. 31, 2019				14,875,035
Beginning balance at Dec. 31, 2018	$ 19,600			$ 15			188,083			(5)	(168,493)
Beginning balance, shares at Dec. 31, 2018	14,748,475			14,748,475
Exercise of stock options, shares	126,560
Unrealized gain (loss) on investments	$ 29
Net loss	(14,385)
Ending balance at Jun. 30, 2019	$ 29,984			$ 28			213,083			24	(183,151)
Ending balance, shares at Jun. 30, 2019	26,713,617			27,810,358
Beginning balance at Mar. 31, 2019	$ 12,847			$ 15			188,811				(175,979)
Beginning balance, shares at Mar. 31, 2019				14,875,035
Stock issued during period, value		$ 21,622	$ 2,203		$ 12	$ 1		$ 21,610	$ 2,202
Stock issued during period, shares					11,838,582	1,096,741
Stock-based compensation expense	460						460
Unrealized gain (loss) on investments	24									24
Net loss	(7,172)										(7,172)
Ending balance at Jun. 30, 2019	$ 29,984			$ 28			$ 213,083			$ 24	$ (183,151)
Ending balance, shares at Jun. 30, 2019	26,713,617			27,810,358