CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated other comprehensive income	Accumulated deficit
Balance at beginning of period (shares) at Dec. 31, 2018		32,538,857
Balance at beginning of period at Dec. 31, 2018	$ 201,576	$ 33	$ 751,109	$ 9,479	$ (559,045)
Increase (Decrease) in Stockholders' Equity
Net loss	(140,880)				(140,880)
Issuance of common stock, net of issuance costs (shares)		4,269,838
Issuance of common stock, net of issuance costs	327,810	$ 4	327,806
Share-based compensation expense	38,826		38,826
Issuance of common stock from ESPP (shares)		9,844
Issuance of common stock from ESPP	323		323
Issuance of common stock under equity incentive plans (shares)		424,144
Issuance of common stock under equity incentive plans	$ 5,372		5,372
Net exercise of warrants (shares)	2,125,000	2,125,000
Net exercise of warrants		$ 2	(2)
Proceeds from disgorgement of shareholders' short-swing profits	$ 1,050		1,050
Unrealized gain (loss) on investments	412			412
Balance at end of period (shares) at Sep. 30, 2019		39,367,683
Balance at end of period at Sep. 30, 2019	434,489	$ 39	1,124,484	9,891	(699,925)
Balance at beginning of period (shares) at Jun. 30, 2019		38,551,426
Balance at beginning of period at Jun. 30, 2019	472,481	$ 39	1,108,306	9,788	(645,652)
Increase (Decrease) in Stockholders' Equity
Net loss	(54,273)				(54,273)
Share-based compensation expense	15,106		15,106
Issuance of common stock from ESPP	0
Issuance of common stock under equity incentive plans (shares)		91,257
Issuance of common stock under equity incentive plans	$ 1,072		1,072
Net exercise of warrants (shares)	725,000	725,000
Unrealized gain (loss) on investments	$ 103			103
Balance at end of period (shares) at Sep. 30, 2019		39,367,683
Balance at end of period at Sep. 30, 2019	$ 434,489	$ 39	1,124,484	9,891	(699,925)
Balance at beginning of period (shares) at Dec. 31, 2019	39,517,329	39,517,329
Balance at beginning of period at Dec. 31, 2019	$ 382,295	$ 40	1,144,667	9,889	(772,301)
Increase (Decrease) in Stockholders' Equity
Net loss	(256,850)				(256,850)
Issuance of common stock, net of issuance costs (shares)		3,538,462
Issuance of common stock, net of issuance costs	323,977	$ 4	323,973
Share-based compensation expense	64,104		64,104
Issuance of common stock from ESPP (shares)		6,207
Issuance of common stock from ESPP	524		524
Issuance of common stock under equity incentive plans (shares)		1,092,468
Issuance of common stock under equity incentive plans	$ 36,284	$ 1	36,283
Net exercise of warrants (shares)	600,000	600,000
Proceeds from disgorgement of shareholders' short-swing profits	$ 41		41
Unrealized gain (loss) on investments	$ 568			568
Balance at end of period (shares) at Sep. 30, 2020	44,482,308	44,754,466
Balance at end of period at Sep. 30, 2020	$ 550,943	$ 45	1,569,592	10,457	(1,029,151)
Balance at beginning of period (shares) at Jun. 30, 2020		44,482,308
Balance at beginning of period at Jun. 30, 2020	605,004	$ 45	1,535,490	11,284	(941,815)
Increase (Decrease) in Stockholders' Equity
Net loss	(87,336)				(87,336)
Share-based compensation expense	21,751		21,751
Issuance of common stock under equity incentive plans (shares)		272,158
Issuance of common stock under equity incentive plans	12,351		12,351
Unrealized gain (loss) on investments	$ (827)			(827)
Balance at end of period (shares) at Sep. 30, 2020	44,482,308	44,754,466
Balance at end of period at Sep. 30, 2020	$ 550,943	$ 45	$ 1,569,592	$ 10,457	$ (1,029,151)