Consolidated Statements of Stockholders' Equity (Unaudited) - 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	1,149		$ 5	1,144
Issuance of common stock upon exercise of stock options (in shares)			512,292
Stock-based compensation expense	6,229			6,229
Unrealized gains (losses) on marketable securities	21				21
Net loss	(47,384)					(47,384)
Ending balance at Mar. 31, 2019	239,996	$ 0	$ 382	582,700	21	(343,107)
Ending balance (in shares) at Mar. 31, 2019		0	38,189,052
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
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	2,569		$ 4	2,565
Issuance of common stock upon exercise of stock options (in shares)			404,298
Stock-based compensation expense	6,994			6,994
Unrealized gains (losses) on marketable securities	652				652
Net loss	(72,807)					(72,807)
Ending balance at Mar. 31, 2020	$ 672,260	$ 0	$ 557	$ 1,231,726	$ 763	$ (560,786)
Ending balance (in shares) at Mar. 31, 2020		0	55,681,027