Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series F Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2016	$ (111,806)			$ 2,536		$ (114,342)
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2016	105,631,019
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2016	$ 129,745
Beginning balance, shares at Dec. 31, 2016			432,413
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 626
Issuance of redeemable convertible preferred stock, net of issuance costs, Shares		483,501
Accretion of redeemable convertible preferred stock to redemption value	(41)			(41)
Accretion of redeemable convertible preferred stock to redemption value	41
Conversion of redeemable convertible preferred stock to common stock	$ 130,412		$ 11	130,401
Conversion of redeemable convertible preferred stock to common stock, Shares	(106,114,520)
Conversion of redeemable convertible preferred stock to common stock	$ (130,412)
Conversion of redeemable convertible preferred stock to common stock, Shares			10,509,774
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	50,009		$ 4	50,005
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares			3,750,000
Exercise of stock options	111			111
Exercise of stock options, shares			31,631
Stock-based compensation expense	1,749			1,749
Unrealized (loss) gain on investments	(33)				$ (33)
Net loss	(22,604)					(22,604)
Ending balance at Dec. 31, 2017	47,797		$ 15	184,761	(33)	(136,946)
Redeemable convertible preferred stock, ending balance at Dec. 31, 2017	$ 0
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2017	0
Ending balance, shares at Dec. 31, 2017	14,723,818		14,723,818
Exercise of stock options	$ 42			42
Exercise of stock options, shares	24,657		24,657
Stock-based compensation expense	$ 3,280			3,280
Unrealized (loss) gain on investments	28				28
Net loss	(31,547)					(31,547)
Ending balance at Dec. 31, 2018	$ 19,600		$ 15	$ 188,083	$ (5)	$ (168,493)
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2018	0
Ending balance, shares at Dec. 31, 2018	14,748,475		14,748,475