CONDENSED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Other Comprehensive Income
Balance, Equity, Shares at Dec. 31, 2017			935,723
Balance, Equity at Dec. 31, 2017	$ (12,528)		$ 1	$ 340	$ (12,869)
Balance, Temporary Equity, Shares at Dec. 31, 2017		7,253,898
Balance, Temporary Equity at Dec. 31, 2017		$ 16,103
Issuance of Series C preferred stock, net of $292,028 of issuance costs		$ 21,391
Issuance of Series C preferred stock, net of $292,028 of issuance costs, Shares		8,118,108
Exercise of common stock options and vesting of founder shares	1			1
Exercise of common stock options and vesting of founder shares, Shares			15,346
Stock-based compensation	88			88
Net loss	(6,193)				(6,193)
Balance, Equity, Shares at Jun. 30, 2018			951,069
Balance, Equity at Jun. 30, 2018	(18,632)		$ 1	429	(19,062)
Balance, Temporary Equity, Shares at Jun. 30, 2018		15,372,006
Balance, Temporary Equity at Jun. 30, 2018		$ 37,494
Balance, Equity, Shares at Dec. 31, 2017			935,723
Balance, Equity at Dec. 31, 2017	(12,528)		$ 1	340	(12,869)
Balance, Temporary Equity, Shares at Dec. 31, 2017		7,253,898
Balance, Temporary Equity at Dec. 31, 2017		$ 16,103
Balance, Equity, Shares at Dec. 31, 2018			31,394,830
Balance, Equity at Dec. 31, 2018	184,360		$ 31	253,807	(69,478)
Balance, Equity, Shares at Mar. 31, 2018			951,069
Balance, Equity at Mar. 31, 2018	(14,691)		$ 1	367	(15,059)
Balance, Temporary Equity, Shares at Mar. 31, 2018		7,253,898
Balance, Temporary Equity at Mar. 31, 2018		$ 16,103
Issuance of Series C preferred stock, net of $292,028 of issuance costs		$ 21,391
Issuance of Series C preferred stock, net of $292,028 of issuance costs, Shares		8,118,108
Stock-based compensation	62			62
Net loss	(4,003)				(4,003)
Balance, Equity, Shares at Jun. 30, 2018			951,069
Balance, Equity at Jun. 30, 2018	(18,632)		$ 1	429	(19,062)
Balance, Temporary Equity, Shares at Jun. 30, 2018		15,372,006
Balance, Temporary Equity at Jun. 30, 2018		$ 37,494
Balance, Equity, Shares at Dec. 31, 2018			31,394,830
Balance, Equity at Dec. 31, 2018	184,360		$ 31	253,807	(69,478)
Exercise of common stock options and vesting of early exercised common stock options	280		$ 1	279
Exercise of common stock options and vesting of early exercised common stock options, Shares			289,864
Share issuance under employee stock purchase plan	247			247
Share issuance under employee stock purchase plan, Shares			26,430
Stock-based compensation	1,272			1,272
Other comprehensive income	228					$ 228
Net loss	(23,221)				(23,221)
Balance, Equity, Shares at Jun. 30, 2019			31,711,124
Balance, Equity at Jun. 30, 2019	163,166		$ 32	255,605	(92,699)	228
Balance, Equity, Shares at Mar. 31, 2019			31,442,454
Balance, Equity at Mar. 31, 2019	174,180		$ 31	254,442	(80,329)	36
Exercise of common stock options and vesting of early exercised common stock options	229		$ 1	228
Exercise of common stock options and vesting of early exercised common stock options, Shares			242,240
Share issuance under employee stock purchase plan	247			247
Share issuance under employee stock purchase plan, Shares			26,430
Stock-based compensation	688			688
Other comprehensive income	192					192
Net loss	(12,370)				(12,370)
Balance, Equity, Shares at Jun. 30, 2019			31,711,124
Balance, Equity at Jun. 30, 2019	$ 163,166		$ 32	$ 255,605	$ (92,699)	$ 228