Condensed Consolidated Statements of Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO [Member]	Jefferies Sales Agreement [Member]	Convertible Preferred Stock (Series A, B, D, E, E-1 and F) [Member]	Convertible Preferred Stock (Series A, B, D, E, E-1 and F) [Member] IPO [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Common Stock [Member] Jefferies Sales Agreement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPO [Member]	Additional Paid-in Capital [Member] Jefferies Sales Agreement [Member]	Accumulated Other Comprehensive Gain [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ (165,833)								$ 8,079				$ (173,912)
Beginning balance, shares at Dec. 31, 2017				118,867,177
Beginning balance at Dec. 31, 2017				$ 173,228
Beginning balance, shares at Dec. 31, 2017						962,898
Issuance of stock, net of issuance costs				$ 68,368
Issuance of stock, net of issuance costs, shares				68,500,000
Stock-based compensation expense	585								585
Vesting of common stock issued upon early exercise of unvested options	3								3
Vesting of common stock issued upon early exercise of unvested options, shares						408
Repayment of promissory notes issued upon early exercise of unvested options	290								290
Repayment of promissory notes issued upon early exercise of unvested options, shares						229,357
Net loss	(12,102)												(12,102)
Ending balance at Mar. 31, 2018	(177,057)								8,957				(186,014)
Ending balance, shares at Mar. 31, 2018				187,367,177
Ending balance at Mar. 31, 2018				$ 241,596
Ending balance, shares at Mar. 31, 2018						1,192,663
Beginning balance at Dec. 31, 2017	(165,833)								8,079				(173,912)
Beginning balance, shares at Dec. 31, 2017				118,867,177
Beginning balance at Dec. 31, 2017				$ 173,228
Beginning balance, shares at Dec. 31, 2017						962,898
Net loss	(39,981)
Ending balance at Sep. 30, 2018	122,563					$ 3			336,453				(213,893)
Ending balance, shares at Sep. 30, 2018						25,784,442
Beginning balance at Dec. 31, 2017	(165,833)								8,079				(173,912)
Beginning balance, shares at Dec. 31, 2017				118,867,177
Beginning balance at Dec. 31, 2017				$ 173,228
Beginning balance, shares at Dec. 31, 2017						962,898
Net loss	(59,900)
Ending balance at Dec. 31, 2018	104,158					$ 3			337,992				(233,837)
Ending balance, shares at Dec. 31, 2018						25,803,135
Beginning balance at Mar. 31, 2018	(177,057)								8,957				(186,014)
Beginning balance, shares at Mar. 31, 2018				187,367,177
Beginning balance at Mar. 31, 2018				$ 241,596
Beginning balance, shares at Mar. 31, 2018						1,192,663
Issuance of stock, net of issuance costs				$ 31,238
Issuance of stock, net of issuance costs, shares				31,250,000
Stock-based compensation expense	687								687
Vesting of common stock issued upon early exercise of unvested options, shares						86
Stock option exercises	96								96
Stock option exercises, shares						22,088
Net loss	(11,941)												(11,941)
Ending balance at Jun. 30, 2018	(188,215)								9,740				(197,955)
Ending balance, shares at Jun. 30, 2018				218,617,177
Ending balance at Jun. 30, 2018				$ 272,834
Ending balance, shares at Jun. 30, 2018						1,214,837
Conversion of convertible preferred stock into common stock upon closing of the initial public offering		$ 272,834					$ 2			$ 272,832
Conversion of convertible preferred stock into common stock upon closing of the initial public offering, shares					(218,617,177)
Conversion of convertible preferred stock into common stock upon closing of the initial public offering					$ (272,834)
Conversion of convertible preferred stock into common stock upon closing of the initial public offering, shares							20,501,927
Conversion of preferred stock warrants to common stock warrants upon closing of initial public offering		364								364
Issuance of stock, net of issuance costs		$ 52,163					$ 1			$ 52,162
Issuance of stock, net of issuance costs, shares							4,000,000
Stock-based compensation expense	1,244								1,244
Vesting of common stock issued upon early exercise of unvested options	1								1
Vesting of common stock issued upon early exercise of unvested options, shares						85
Exercise of common stock warrant	79								79
Exercise of common stock warrant, shares						51,032
Stock option exercises	31								31
Stock option exercises, shares						16,561
Net loss	(15,938)												(15,938)
Ending balance at Sep. 30, 2018	122,563					$ 3			336,453				(213,893)
Ending balance, shares at Sep. 30, 2018						25,784,442
Beginning balance at Dec. 31, 2018	104,158					$ 3			337,992				(233,837)
Beginning balance, shares at Dec. 31, 2018						25,803,135
Stock-based compensation expense	1,313								1,313
Vesting of common stock issued upon early exercise of unvested options, shares						85
Stock option exercises	21								21
Stock option exercises, shares						3,754
Unrealized gain on marketable securities	9											$ 9
Net loss	(19,426)												(19,426)
Ending balance at Mar. 31, 2019	86,075					$ 3			339,326			9	(253,263)
Ending balance, shares at Mar. 31, 2019						25,806,974
Beginning balance at Dec. 31, 2018	104,158					$ 3			337,992				(233,837)
Beginning balance, shares at Dec. 31, 2018						25,803,135
Issuance of stock, net of issuance costs, shares								39,506
Net loss	(61,342)
Ending balance at Sep. 30, 2019	48,152					$ 3			343,327			1	(295,179)
Ending balance, shares at Sep. 30, 2019						25,866,547
Beginning balance at Mar. 31, 2019	86,075					$ 3			339,326			9	(253,263)
Beginning balance, shares at Mar. 31, 2019						25,806,974
Stock-based compensation expense	1,797								1,797
Vesting of common stock issued upon early exercise of unvested options, shares						85
Stock option exercises	77								77
Stock option exercises, shares						12,364
Unrealized gain on marketable securities	2											2
Net loss	(20,767)												(20,767)
Ending balance at Jun. 30, 2019	67,184					$ 3			341,200			11	(274,030)
Ending balance, shares at Jun. 30, 2019						25,819,423
Issuance of stock, net of issuance costs			$ 145								$ 145
Issuance of stock, net of issuance costs, shares								39,506
Stock-based compensation expense	1,948								1,948
Vesting of common stock issued upon early exercise of unvested options, shares						85
Stock option exercises	34								34
Stock option exercises, shares						7,533
Unrealized gain on marketable securities	(10)											(10)
Net loss	(21,149)												(21,149)
Ending balance at Sep. 30, 2019	$ 48,152					$ 3			$ 343,327			$ 1	$ (295,179)
Ending balance, shares at Sep. 30, 2019						25,866,547