CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (UNAUDITED) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	$ (39,454)			$ 800	$ (73)	$ (40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Vesting of common stock from option exercises	16			16
Vesting of common stock from option exercises, Shares			34,137
Issuance of common stock from option exercises	69			69
Issuance of common stock from option exercises, Shares			26,204
Stock-based compensation	380			380
Other comprehensive (loss) gain	(13)				(13)
Net loss	(10,217)					(10,217)
Ending balance at Mar. 31, 2018	(49,219)			1,265	(86)	(50,398)
Ending balance, Shares at Mar. 31, 2018		127,199,705
Ending balance at Mar. 31, 2018		$ 137,762
Ending balance, Shares at Mar. 31, 2018			2,697,352
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	(39,454)			800	(73)	(40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Other comprehensive (loss) gain	52
Net loss	(36,791)
Ending balance at Sep. 30, 2018	214,183		$ 4	291,171	(20)	(76,972)
Ending balance, Shares at Sep. 30, 2018			37,310,063
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	(39,454)			800	(73)	(40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Net loss	(55,577)
Ending balance at Dec. 31, 2018	196,355		$ 3	292,187	(77)	(95,758)
Ending balance, Shares at Dec. 31, 2018			37,358,526
Balance balance at Mar. 31, 2018		$ 137,762
Beginning balance at Mar. 31, 2018	(49,219)			1,265	(86)	(50,398)
Beginning balance, Shares at Mar. 31, 2018		127,199,705
Beginning balance, Shares at Mar. 31, 2018			2,697,352
Conversion of convertible preferred stock into common stock upon initial public offering	137,762	$ (137,762)	$ 3	137,759
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		127,199,705
Conversion of convertible preferred stock into common stock upon initial public offering, Shares			24,168,656
Issuance of common stock in initial public offering, net of discounts and issuance costs	150,843		$ 1	150,842
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares			10,350,000
Vesting of common stock from option exercises	(10)			(10)
Vesting of common stock from option exercises, Shares			27,364
Issuance of common stock from option exercises	16			16
Issuance of common stock from option exercises, Shares			11,530
Stock-based compensation	569			569
Other comprehensive (loss) gain	38				38
Net loss	(12,426)					(12,426)
Ending balance at Jun. 30, 2018	227,573		$ 4	290,441	(48)	(62,824)
Ending balance, Shares at Jun. 30, 2018			37,254,902
Vesting of common stock from option exercises	51			51
Vesting of common stock from option exercises, Shares			35,749
Issuance of common stock from option exercises	(40)			(40)
Issuance of common stock from option exercises, Shares			19,412
Stock-based compensation	719			719
Other comprehensive (loss) gain	28				28
Net loss	(14,148)					(14,148)
Ending balance at Sep. 30, 2018	214,183		$ 4	291,171	(20)	(76,972)
Ending balance, Shares at Sep. 30, 2018			37,310,063
Beginning balance at Dec. 31, 2018	196,355		$ 3	292,187	(77)	(95,758)
Beginning balance, Shares at Dec. 31, 2018			37,358,526
Vesting of common stock from option exercises	18			18
Vesting of common stock from option exercises, Shares			31,584
Issuance of common stock from option exercises	21			21
Issuance of common stock from option exercises, Shares			18,876
Issuance of common stock pursuant to employee stock purchase plan	396			396
Issuance of common stock pursuant to employee stock purchase plan, Shares			28,311
Stock-based compensation	1,243			1,243
Other comprehensive (loss) gain	83				83
Net loss	(23,852)					(23,852)
Ending balance at Mar. 31, 2019	174,264		$ 3	293,865	6	(119,610)
Ending balance, Shares at Mar. 31, 2019			37,437,297
Beginning balance at Dec. 31, 2018	$ 196,355		$ 3	292,187	(77)	(95,758)
Beginning balance, Shares at Dec. 31, 2018			37,358,526
Issuance of common stock from option exercises, Shares	74,709
Other comprehensive (loss) gain	$ 331
Net loss	(79,749)
Ending balance at Sep. 30, 2019	257,822		$ 4	433,071	254	(175,507)
Ending balance, Shares at Sep. 30, 2019			43,956,070
Beginning balance at Mar. 31, 2019	174,264		$ 3	293,865	6	(119,610)
Beginning balance, Shares at Mar. 31, 2019			37,437,297
Issuance of common stock in follow-on offering, net of discounts and issuance costs	134,525		$ 1	134,524
Issuance of common stock in follow-on offering, net of discounts and issuance costs, Shares			6,388,889
Vesting of common stock from option exercises	18			18
Vesting of common stock from option exercises, Shares			31,024
Issuance of common stock from option exercises	55			55
Issuance of common stock from option exercises, Shares			12,404
Stock-based compensation	1,970			1,970
Other comprehensive (loss) gain	180				180
Net loss	(26,270)					(26,270)
Ending balance at Jun. 30, 2019	284,742		$ 4	430,432	186	(145,880)
Ending balance, Shares at Jun. 30, 2019			43,869,614
Vesting of common stock from option exercises	11			11
Vesting of common stock from option exercises, Shares			16,104
Issuance of common stock from option exercises	209			209
Issuance of common stock from option exercises, Shares			43,429
Issuance of common stock pursuant to employee stock purchase plan	433			433
Issuance of common stock pursuant to employee stock purchase plan, Shares			26,923
Stock-based compensation	1,986			1,986
Other comprehensive (loss) gain	68				68
Net loss	(29,627)					(29,627)
Ending balance at Sep. 30, 2019	$ 257,822		$ 4	$ 433,071	$ 254	$ (175,507)
Ending balance, Shares at Sep. 30, 2019			43,956,070