Condensed Statements of Redeemable Convertible Preferred Stock, Convertible Preferred Stock and Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance, shares at Dec. 31, 2017		12,956,724	69,834,789
Beginning balance at Dec. 31, 2017		$ 50,367	$ 122,990
Beginning balance, shares at Dec. 31, 2017				8,287,983
Beginning balance at Dec. 31, 2017	$ (176,986)				$ 4,591	$ (6)	$ (181,571)
Accretion of redeemable convertible preferred stock to redemption value	(1,109)				(1,109)
Accretion of redeemable convertible preferred stock to redemption value		$ 1,109
Issuance of common stock upon exercise of options	39				39
Issuance of common stock upon exercise of options, shares				18,621
Stock-based compensation expense	545				545
Other comprehensive income	6					6
Net loss	(14,106)						(14,106)
Ending balance, shares at Mar. 31, 2018		12,956,724	69,834,789
Ending balance at Mar. 31, 2018		$ 51,476	$ 122,990
Ending balance, shares at Mar. 31, 2018				8,306,604
Ending balance at Mar. 31, 2018	(191,611)				4,066		(195,677)
Beginning balance, shares at Dec. 31, 2017		12,956,724	69,834,789
Beginning balance at Dec. 31, 2017		$ 50,367	$ 122,990
Beginning balance, shares at Dec. 31, 2017				8,287,983
Beginning balance at Dec. 31, 2017	(176,986)				4,591	(6)	(181,571)
Net loss	(31,701)
Ending balance, shares at Jun. 30, 2018		31,053,601	71,832,498
Ending balance at Jun. 30, 2018		$ 144,910	$ 133,192
Ending balance, shares at Jun. 30, 2018				8,324,106
Ending balance at Jun. 30, 2018	(210,213)				3,059		(213,272)
Beginning balance, shares at Mar. 31, 2018		12,956,724	69,834,789
Beginning balance at Mar. 31, 2018		$ 51,476	$ 122,990
Beginning balance, shares at Mar. 31, 2018				8,306,604
Beginning balance at Mar. 31, 2018	(191,611)				4,066		(195,677)
Issuance of convertible preferred stock upon conversion of notes, net of issuance costs		$ 5,452	$ 10,202
Issuance of convertible preferred stock upon conversion of notes, net of issuance costs, shares		1,067,550	1,997,709
Loss on extinguishment of convertible notes	(370)				(370)
Issuance of convertible preferred stock, net of issuance costs		$ 86,640
Issuance of convertible preferred stock, net of issuance costs, shares		17,029,327
Accretion of redeemable convertible preferred stock to redemption value	(1,342)				(1,342)
Accretion of redeemable convertible preferred stock to redemption value		$ 1,342
Issuance of common stock upon exercise of options	24				24
Issuance of common stock upon exercise of options, shares				17,502
Stock-based compensation expense	681				681
Net loss	(17,595)						(17,595)
Ending balance, shares at Jun. 30, 2018		31,053,601	71,832,498
Ending balance at Jun. 30, 2018		$ 144,910	$ 133,192
Ending balance, shares at Jun. 30, 2018				8,324,106
Ending balance at Jun. 30, 2018	$ (210,213)				3,059		(213,272)
Beginning balance, shares at Dec. 31, 2018	104,778,246	31,053,601	73,724,645
Beginning balance at Dec. 31, 2018	$ 294,200	$ 151,381	$ 142,819
Beginning balance, shares at Dec. 31, 2018				8,593,077
Beginning balance at Dec. 31, 2018	(257,690)					(25)	(257,665)
Issuance of convertible preferred stock, net of issuance costs		$ 43,572	$ 26,279
Issuance of convertible preferred stock, net of issuance costs, shares		6,350,345	3,831,766
Accretion of redeemable convertible preferred stock to redemption value	3,355				(3,260)		(95)
Accretion of redeemable convertible preferred stock to redemption value		$ 3,355
Compensation expense related to stock sales by current and former employees	819				819
Issuance of common stock upon exercise of options	1,319				1,319
Issuance of common stock upon exercise of options, shares				739,053
Issuance of common stock upon exercise of warrants, shares				4,935
Issuance of common stock upon exercise of warrants	13				13
Stock-based compensation expense	1,109				1,109
Other comprehensive income	28					28
Net loss	(23,222)						(23,222)
Ending balance, shares at Mar. 31, 2019		37,403,946	77,556,411
Ending balance at Mar. 31, 2019		$ 198,308	$ 169,098
Ending balance, shares at Mar. 31, 2019				9,337,065
Ending balance at Mar. 31, 2019	$ (280,979)					3	(280,982)
Beginning balance, shares at Dec. 31, 2018	104,778,246	31,053,601	73,724,645
Beginning balance at Dec. 31, 2018	$ 294,200	$ 151,381	$ 142,819
Beginning balance, shares at Dec. 31, 2018				8,593,077
Beginning balance at Dec. 31, 2018	(257,690)					(25)	(257,665)
Net loss	$ (50,097)
Ending balance, shares at Jun. 30, 2019	114,960,357	37,403,946	77,556,411
Ending balance at Jun. 30, 2019	$ 367,330	$ 198,228	$ 169,102
Ending balance, shares at Jun. 30, 2019				9,701,266
Ending balance at Jun. 30, 2019	(306,122)			$ 1	1,729	5	(307,857)
Beginning balance, shares at Mar. 31, 2019		37,403,946	77,556,411
Beginning balance at Mar. 31, 2019		$ 198,308	$ 169,098
Beginning balance, shares at Mar. 31, 2019				9,337,065
Beginning balance at Mar. 31, 2019	(280,979)					3	(280,982)
Stock issuance costs		$ (80)	$ 4
Issuance of common stock upon exercise of options	423			$ 1	422
Issuance of common stock upon exercise of options, shares				358,459
Issuance of common stock upon exercise of warrants, shares				5,742
Issuance of common stock upon exercise of warrants	20				20
Stock-based compensation expense	1,287				1,287
Other comprehensive income	2					2
Net loss	$ (26,875)						(26,875)
Ending balance, shares at Jun. 30, 2019	114,960,357	37,403,946	77,556,411
Ending balance at Jun. 30, 2019	$ 367,330	$ 198,228	$ 169,102
Ending balance, shares at Jun. 30, 2019				9,701,266
Ending balance at Jun. 30, 2019	$ (306,122)			$ 1	$ 1,729	$ 5	$ (307,857)