Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 279,981	$ 0	$ 377	$ 575,327	$ 0	$ (295,723)
Beginning balance (in shares) at Dec. 31, 2018		0	37,676,760
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options	1,230		$ 5	1,225
Issuance of common stock upon exercise of stock options (in shares)			538,348
Stock-based compensation expense	10,336			10,336
Unrealized gains (losses) on marketable securities	175				175
Net loss	(68,844)					(68,844)
Ending balance at Jun. 30, 2019	222,878	$ 0	$ 382	586,888	175	(364,567)
Ending balance (in shares) at Jun. 30, 2019		0	38,215,108
Beginning balance at Dec. 31, 2018	279,981	$ 0	$ 377	575,327	0	(295,723)
Beginning balance (in shares) at Dec. 31, 2018		0	37,676,760
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(192,300)
Ending balance at Dec. 31, 2019	546,467	$ 0	$ 516	1,033,819	111	(487,979)
Ending balance (in shares) at Dec. 31, 2019		0	51,617,639
Beginning balance at Mar. 31, 2019	239,996	$ 0	$ 382	582,700	21	(343,107)
Beginning balance (in shares) at Mar. 31, 2019		0	38,189,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options	81		$ 0	81
Issuance of common stock upon exercise of stock options (in shares)			26,056
Stock-based compensation expense	4,107			4,107
Unrealized gains (losses) on marketable securities	154				154
Net loss	(21,460)					(21,460)
Ending balance at Jun. 30, 2019	222,878	$ 0	$ 382	586,888	175	(364,567)
Ending balance (in shares) at Jun. 30, 2019		0	38,215,108
Beginning balance at Dec. 31, 2019	546,467	$ 0	$ 516	1,033,819	111	(487,979)
Beginning balance (in shares) at Dec. 31, 2019		0	51,617,639
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs	188,385		$ 37	188,348
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)			3,659,090
Issuance of common stock upon exercise of stock options	7,396		$ 8	7,388
Issuance of common stock upon exercise of stock options (in shares)			805,264
Stock-based compensation expense	17,603			17,603
Unrealized gains (losses) on marketable securities	(119)				(119)
Net loss	(140,048)					(140,048)
Ending balance at Jun. 30, 2020	619,684	$ 0	$ 561	1,247,158	(8)	(628,027)
Ending balance (in shares) at Jun. 30, 2020		0	56,081,993
Beginning balance at Mar. 31, 2020	672,260	$ 0	$ 557	1,231,726	763	(560,786)
Beginning balance (in shares) at Mar. 31, 2020		0	55,681,027
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options	4,827		$ 4	4,823
Issuance of common stock upon exercise of stock options (in shares)			400,966
Stock-based compensation expense	10,609			10,609
Unrealized gains (losses) on marketable securities	(771)				(771)
Net loss	(67,241)					(67,241)
Ending balance at Jun. 30, 2020	$ 619,684	$ 0	$ 561	$ 1,247,158	$ (8)	$ (628,027)
Ending balance (in shares) at Jun. 30, 2020		0	56,081,993