CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Series A, B-1, B-2 and C Convertible Preferred Shares
Convertible preferred shares, beginning balance (in shares) at Dec. 31, 2016						3,632,711
Convertible preferred shares, beginning balance at Dec. 31, 2016						$ 192,667
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series C convertible preferred shares, net of issuance costs (in shares)						690,333
Issuance of Series C convertible preferred shares, net of issuance costs of $429	$ 0					$ 51,871
Effect of Conversion (Note 1), convertible preferred shares (in shares)						(4,323,044)
Effect of Conversion (Note 1), convertible preferred shares						$ (244,538)
Convertible preferred shares, ending balance (in shares) at Dec. 31, 2017						0
Convertible preferred shares, ending balance at Dec. 31, 2017						$ 0
Beginning balance (in shares) at Dec. 31, 2016		0
Beginning balance at Dec. 31, 2016	(139,760)	$ 0	$ 5,825	$ 0	$ (145,585)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of Conversion (Note 1) (in shares)		24,425,190
Effect of Conversion (Note 1)	244,538	$ 244	244,294
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)		8,166,496
Issuance of common stock sold in public offering, net of underwriting discounts, commissions, and offering costs	$ 124,613	$ 82	124,531
Issuance of common stock upon exercise of stock options (in shares)	0
Stock-based compensation expense	$ 4,866		4,866
Net loss	(50,284)				(50,284)
Ending balance (in shares) at Dec. 31, 2017		32,591,686
Ending balance at Dec. 31, 2017	183,973	$ 326	379,516	0	(195,869)
Convertible preferred shares, ending balance (in shares) at Dec. 31, 2018						0
Convertible preferred shares, ending balance at Dec. 31, 2018						$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)		4,945,000
Issuance of common stock sold in public offering, net of underwriting discounts, commissions, and offering costs	185,259	$ 50	185,209
Issuance of common stock upon exercise of stock options (in shares)		140,074
Issuance of common stock upon exercise of stock options	915	$ 1	914
Stock-based compensation expense	9,688		9,688
Net loss	(99,854)				(99,854)
Ending balance (in shares) at Dec. 31, 2018		37,676,760
Ending balance at Dec. 31, 2018	279,981	$ 377	575,327	0	(295,723)
Convertible preferred shares, ending balance (in shares) at Dec. 31, 2019						0
Convertible preferred shares, ending balance at Dec. 31, 2019						$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)		12,432,431
Issuance of common stock sold in public offering, net of underwriting discounts, commissions, and offering costs	$ 431,780	$ 124	431,656
Issuance of common stock upon exercise of stock options (in shares)	1,504,583	1,508,448
Issuance of common stock upon exercise of stock options	$ 6,441	$ 15	6,426
Stock-based compensation expense	20,410		20,410
Unrealized gains (losses) on marketable securities	111			111
Net loss	(192,256)				(192,256)
Ending balance (in shares) at Dec. 31, 2019		51,617,639
Ending balance at Dec. 31, 2019	$ 546,467	$ 516	$ 1,033,819	$ 111	$ (487,979)