Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		45,563,693
Beginning balance at Dec. 31, 2018	$ 60,632	$ 5	$ 1,165,003	$ (755)	$ (1,103,621)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(78,234)				(78,234)
Unrealized gain (loss) from available-for-sale debt securities	710			710
Vesting of restricted shares (in shares)		75,331
Vesting of restricted shares	0
Exercise of options (in shares)		341,337
Exercise of options	2,869		2,869
Exercise of warrants (in shares)		81,104
Exercise of warrants	1,000		1,000
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		63,732
Issuance of common stock upon purchase by employee stock purchase plan	1,027		1,027
Share-based compensation expense	11,862		11,862
Ending balance (in shares) at Jun. 30, 2019		46,125,197
Ending balance at Jun. 30, 2019	(134)	$ 5	1,181,761	(45)	(1,181,855)
Beginning balance (in shares) at Mar. 31, 2019		45,967,080
Beginning balance at Mar. 31, 2019	28,004	$ 5	1,174,661	(281)	(1,146,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(35,474)				(35,474)
Unrealized gain (loss) from available-for-sale debt securities	236			236
Vesting of restricted shares (in shares)		2,218
Vesting of restricted shares	0
Exercise of options (in shares)		155,899
Exercise of options	1,352		1,352
Share-based compensation expense related to share-based awards for employee stock purchase plan	255		255
Share-based compensation expense	5,493		5,493
Ending balance (in shares) at Jun. 30, 2019		46,125,197
Ending balance at Jun. 30, 2019	(134)	$ 5	1,181,761	(45)	(1,181,855)
Beginning balance (in shares) at Dec. 31, 2019		46,189,870
Beginning balance at Dec. 31, 2019	(42,279)	$ 5	1,194,327	3	(1,236,614)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(81,534)				(81,534)
Unrealized gain (loss) from available-for-sale debt securities	105			105
Vesting of restricted shares (in shares)		203,324
Vesting of restricted shares	0
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		55,297
Issuance of common stock upon purchase by employee stock purchase plan	990		990
Share-based compensation expense	13,299		13,299
Ending balance (in shares) at Jun. 30, 2020		46,448,491
Ending balance at Jun. 30, 2020	(109,419)	$ 5	1,208,616	108	(1,318,148)
Beginning balance (in shares) at Mar. 31, 2020		46,387,437
Beginning balance at Mar. 31, 2020	(74,153)	$ 5	1,200,776	(666)	(1,274,268)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(43,880)				(43,880)
Unrealized gain (loss) from available-for-sale debt securities	774			774
Vesting of restricted shares (in shares)		61,054
Vesting of restricted shares	0
Share-based compensation expense	7,840		7,840
Ending balance (in shares) at Jun. 30, 2020		46,448,491
Ending balance at Jun. 30, 2020	$ (109,419)	$ 5	$ 1,208,616	$ 108	$ (1,318,148)