CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCKAND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Redeemable Convertible Preferred Stock (in shares)	61,279,079
Redeemable Convertible Preferred Stock	$ 166,486
Beginning balance (in shares) at Dec. 31, 2017		33,740,323
Beginning balance at Dec. 31, 2017	(31,367)	$ 3	$ 92,222	$ (123,592)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and common stock warrants (in shares)		3,204,994
Issuance of common stock upon exercise of stock options	7,012	$ 1	7,011
Stock-based compensation expense	5,667		5,667
Net loss	(14,542)			(14,542)
Ending balance (in shares) at Sep. 30, 2018		36,945,317
Ending balance at Sep. 30, 2018	$ (33,230)	$ 4	104,900	(138,134)
Redeemable Convertible Preferred Stock (in shares)	61,279,079
Redeemable Convertible Preferred Stock	$ 166,486
Beginning balance (in shares) at Jun. 30, 2018		35,880,003
Beginning balance at Jun. 30, 2018	(30,611)	$ 4	100,173	(130,788)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and common stock warrants (in shares)		1,065,314
Issuance of common stock upon exercise of stock options	2,741		2,741
Stock-based compensation expense	1,986		1,986
Net loss	(7,346)			(7,346)
Ending balance (in shares) at Sep. 30, 2018		36,945,317
Ending balance at Sep. 30, 2018	$ (33,230)	$ 4	104,900	(138,134)
Redeemable Convertible Preferred Stock (in shares)	61,279,079
Redeemable Convertible Preferred Stock	$ 166,486
Redeemable Convertible Preferred Stock (in shares)	0
Redeemable Convertible Preferred Stock	$ 0
Beginning balance (in shares) at Dec. 31, 2018		106,454,321
Beginning balance at Dec. 31, 2018	$ 243,745	$ 11	387,233	(143,499)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and common stock warrants (in shares)	4,707,617	5,041,302
Issuance of common stock upon exercise of stock options	$ 14,145		14,145
Stock-based compensation expense	10,714		10,714
Issuance of common stock for settlement of RSUs (in shares)		123,562
Issuance of common stock in connection with employee stock purchase plan (in shares)		280,308
Issuance of common stock in connection with employee stock purchase plan	3,577		3,577
Net loss	(9,582)			(9,582)
Ending balance (in shares) at Sep. 30, 2019		111,899,493
Ending balance at Sep. 30, 2019	$ 262,599	$ 11	415,669	(153,081)
Redeemable Convertible Preferred Stock (in shares)	0
Redeemable Convertible Preferred Stock	$ 0
Beginning balance (in shares) at Jun. 30, 2019		110,708,530
Beginning balance at Jun. 30, 2019	257,633	$ 11	407,876	(150,254)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and common stock warrants (in shares)		1,122,763
Issuance of common stock upon exercise of stock options	3,795		3,795
Stock-based compensation expense	3,998		3,998
Issuance of common stock for settlement of RSUs (in shares)		68,200
Net loss	(2,827)			(2,827)
Ending balance (in shares) at Sep. 30, 2019		111,899,493
Ending balance at Sep. 30, 2019	$ 262,599	$ 11	$ 415,669	$ (153,081)
Redeemable Convertible Preferred Stock (in shares)	0
Redeemable Convertible Preferred Stock	$ 0