Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income/ (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 370,567	$ 4	$ 573,183	$ (67)	$ (202,553)
Beginning balance, shares at Dec. 31, 2018		40,288,949
Issuance of common stock in connection with the2019 follow-on offering, net of issuance costs of $17,638	271,213	$ 1	271,212
Issuance of common stock in connection with the2019 follow-on offering, net of issuance costs of $17,638, shares		5,663,750
Issuance of common stock upon the exercise of options and release of stock awards	280		280
Issuance of common stock upon the exercise of options and release of stock awards, shares		49,076
Vesting of early exercised stock options	8		8
Stock-based compensation	6,981		6,981
Unrealized gains	363			363
Net loss	(37,470)				(37,470)
Ending balance at Mar. 31, 2019	611,942	$ 5	851,664	296	(240,023)
Ending balance, shares at Mar. 31, 2019		46,001,775
Beginning balance at Dec. 31, 2019	406,274	$ 5	884,486	549	(478,766)
Beginning balance, shares at Dec. 31, 2019		46,379,073
Issuance of common stock upon the exercise of options and release of stock awards	$ 1,820		1,820
Issuance of common stock upon the exercise of options and release of stock awards, shares	107,870	233,113
Stock-based compensation	$ 10,752		10,752
Unrealized gains	122			122
Net loss	(69,868)				(69,868)
Ending balance at Mar. 31, 2020	$ 349,100	$ 5	$ 897,058	$ 671	$ (548,634)
Ending balance, shares at Mar. 31, 2020		46,612,186