STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series C Preferred Stock	Series D Preferred Stock	Series A Preferred Stock	Common Stock	Additional Paid-in Capital	AOCI Attributable to Parent	Retained Earnings
Beginning balance (in shares) at Dec. 31, 2016	63,361,895
Beginning balance at Dec. 31, 2016	$ 66,883
Beginning balance (in shares) at Dec. 31, 2016					2,256,530
Beginning balance at Dec. 31, 2016	(47,628)				$ 2	$ 466	$ 0	$ (48,096)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation (in shares)		16,274,192	24,493,615
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation		$ 22,882	$ 57,305
Issuance of common stock upon exercise of stock options (in shares)					16,079
Issuance of common stock upon exercise of stock options	14					14
Stock-based compensation	876					876
Net unrealized gain (loss) on available-for-sale-investments, net of tax	(13)						(13)
Net loss	$ (41,290)							(41,290)
Ending balance (in shares) at Dec. 31, 2017	104,129,702
Ending balance at Dec. 31, 2017	$ 147,070
Ending balance (in shares) at Dec. 31, 2017					2,272,609
Ending balance at Dec. 31, 2017	(88,041)				$ 2	1,356	(13)	(89,386)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	$ 5,152					5,152
Issuance of Series A warrant for cash (in shares)	95,936
Issuance of Series A convertible preferred stock upon exercise of warrant	$ 458
Conversion of convertible preferred shares to common stock (in shares)				(104,225,638)
Conversion of convertible preferred shares to common stock				$ (147,528)
Common stock issued in conversion of convertible preferred stock (in shares)					26,187,321
Conversion of convertible preferred stock to common stock	147,528				$ 26	147,502
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs (in shares0					13,455,000
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs	231,186				$ 13	231,173
Reclassification of common stock warrant liability to equity	194					194
Issuance of warrant in connection with Term Loan	884					884
Issuance of common stock upon exercise of stock options, warrants and employee stock purchase plan (in shares)					233,317
Issuance of common stock under equity incentive plans	570				$ 1	569
Net unrealized gain (loss) on available-for-sale-investments, net of tax	(140)						(140)
Net loss	$ (102,808)							(102,808)
Ending balance (in shares) at Dec. 31, 2018	0
Ending balance at Dec. 31, 2018	$ 0
Ending balance (in shares) at Dec. 31, 2018	42,148,247				42,148,247
Ending balance at Dec. 31, 2018	$ 194,525				$ 42	386,830	(153)	(192,194)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	1,108,700
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs (in shares0					6,440,000
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs	$ 217,009				$ 6	217,003
Issuance of warrant in connection with Term Loan	535					535
Issuance of common stock under equity incentive plans (in shares)					1,174,929
Issuance of common stock under equity incentive plans	3,113				$ 2	3,111
Stock-based compensation	25,168					25,168
Net unrealized gain (loss) on available-for-sale-investments, net of tax	346						346
Net loss	$ (176,813)							(176,813)
Ending balance (in shares) at Dec. 31, 2019	0
Ending balance at Dec. 31, 2019	$ 0
Ending balance (in shares) at Dec. 31, 2019	49,763,176				49,763,176
Ending balance at Dec. 31, 2019	$ 263,883				$ 50	$ 632,647	$ 193	$ (369,007)