Condensed Statements of Stockholders' (Deficit) Equity (Unaudited) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Note Receivable from Stockholder	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balances at Dec. 31, 2017	$ (109,001)			$ (208)	$ 6,218		$ (115,011)
Temporary Equity Balance, Shares at Dec. 31, 2017		173,750,421
Temporary Equity Balance at Dec. 31, 2017		$ 102,505
Common Stock Balance, Shares at Dec. 31, 2017			465,330
Exercise of common stock options	38				38
Stock-based compensation expense	249				249
Vesting of early exercised shares	14				14
Net loss	(12,046)						(12,046)
Balances at Mar. 31, 2018	(120,746)			(208)	6,519		(127,057)
Temporary Equity Balance, Shares at Mar. 31, 2018		173,750,421
Temporary Equity Balance at Mar. 31, 2018		$ 102,505
Common Stock Balance, Shares at Mar. 31, 2018			471,949
Exercise of common stock options, Shares			6,619
Balances at Dec. 31, 2017	(109,001)			(208)	6,218		(115,011)
Temporary Equity Balance, Shares at Dec. 31, 2017		173,750,421
Temporary Equity Balance at Dec. 31, 2017		$ 102,505
Common Stock Balance, Shares at Dec. 31, 2017			465,330
Net unrealized gain on available-for- sale securities	0
Net loss	(23,587)
Balances at Jun. 30, 2018	(132,019)			(208)	6,787		(138,598)
Temporary Equity Balance, Shares at Jun. 30, 2018		173,750,421
Temporary Equity Balance at Jun. 30, 2018		$ 135,720
Common Stock Balance, Shares at Jun. 30, 2018			479,377
Balances at Dec. 31, 2017	(109,001)			(208)	6,218		(115,011)
Temporary Equity Balance, Shares at Dec. 31, 2017		173,750,421
Temporary Equity Balance at Dec. 31, 2017		$ 102,505
Common Stock Balance, Shares at Dec. 31, 2017			465,330
Balances at Dec. 31, 2018	131,539		$ 23		281,891	$ (47)	(150,328)
Common Stock Balance, Shares at Dec. 31, 2018			22,848,184
Balances at Mar. 31, 2018	(120,746)			(208)	6,519		(127,057)
Temporary Equity Balance, Shares at Mar. 31, 2018		173,750,421
Temporary Equity Balance at Mar. 31, 2018		$ 102,505
Common Stock Balance, Shares at Mar. 31, 2018			471,949
Issuance of Series E redeemable convertible preferred stock, net of issuance costs		$ 33,215
Exercise of common stock options	36				36
Stock-based compensation expense	232				232
Net unrealized gain on available-for- sale securities	0
Net loss	(11,541)						(11,541)
Balances at Jun. 30, 2018	(132,019)			$ (208)	6,787		(138,598)
Temporary Equity Balance, Shares at Jun. 30, 2018		173,750,421
Temporary Equity Balance at Jun. 30, 2018		$ 135,720
Common Stock Balance, Shares at Jun. 30, 2018			479,377
Exercise of common stock options, Shares			7,428
Balances at Dec. 31, 2018	131,539		$ 23		281,891	(47)	(150,328)
Common Stock Balance, Shares at Dec. 31, 2018			22,848,184
Exercise of common stock options	42		$ 8,347		42
Issuance of common stock under Employee Stock Purchase Plan	671				671
Issuance of common stock under Employee Stock Purchase Plan, Shares			68,910
Stock-based compensation expense	2,286				2,286
Net unrealized gain on available-for- sale securities	103					103
Adoption of new accounting standards at Mar. 31, 2019	10,327						10,327
Net loss	(14,250)						(14,250)
Balances at Mar. 31, 2019	130,718		$ 23		284,890	56	(154,251)
Common Stock Balance, Shares at Mar. 31, 2019			22,925,441
Balances at Dec. 31, 2018	131,539		$ 23		281,891	(47)	(150,328)
Common Stock Balance, Shares at Dec. 31, 2018			22,848,184
Net unrealized gain on available-for- sale securities	260
Net loss	(28,043)
Balances at Jun. 30, 2019	119,555		$ 23		287,363	213	(168,044)
Common Stock Balance, Shares at Jun. 30, 2019			22,927,203
Balances at Mar. 31, 2019	130,718		$ 23		284,890	56	(154,251)
Common Stock Balance, Shares at Mar. 31, 2019			22,925,441
Exercise of common stock options	10		$ 1,762		10
Stock-based compensation expense	2,463				2,463
Net unrealized gain on available-for- sale securities	157					157
Net loss	(13,793)						(13,793)
Balances at Jun. 30, 2019	$ 119,555		$ 23		$ 287,363	$ 213	$ (168,044)
Common Stock Balance, Shares at Jun. 30, 2019			22,927,203