CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at beginning of period at Dec. 31, 2018		$ 507,366,000	$ (69,000)	$ (292,173,000)	$ 215,124,000
Balance at beginning of period (in shares) at Dec. 31, 2018	41,210,725
Increase (Decrease) in Stockholders' Equity
Vesting of restricted stock units		(188,000)			(188,000)
Vesting of restricted stock units, shares	58,918
Unrealized gain (loss) on marketable securities			34,000		34,000
Foreign currency translation adjustment			(14,000)		(14,000)
Stock-based compensation expense		4,862,000			4,862,000
Net loss				(37,565,000)	(37,565,000)
Balance at end of period at Mar. 31, 2019		512,040,000	(49,000)	(329,738,000)	182,253,000
Balance at end of period (in shares) at Mar. 31, 2019	41,269,643
Balance at beginning of period at Dec. 31, 2018		507,366,000	(69,000)	(292,173,000)	215,124,000
Balance at beginning of period (in shares) at Dec. 31, 2018	41,210,725
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities					64,000
Net loss					(87,441,000)
Balance at end of period at Jun. 30, 2019		516,836,000	8,000	(379,614,000)	137,230,000
Balance at end of period (in shares) at Jun. 30, 2019	41,278,570
Balance at beginning of period at Mar. 31, 2019		512,040,000	(49,000)	(329,738,000)	182,253,000
Balance at beginning of period (in shares) at Mar. 31, 2019	41,269,643
Increase (Decrease) in Stockholders' Equity
Vesting of restricted stock units		(18,000)			(18,000)
Vesting of restricted stock units, shares	8,927
Unrealized gain (loss) on marketable securities			30,000		30,000
Foreign currency translation adjustment			27,000		27,000
Stock-based compensation expense		4,814,000			4,814,000
Net loss				(49,876,000)	(49,876,000)
Balance at end of period at Jun. 30, 2019		516,836,000	8,000	(379,614,000)	137,230,000
Balance at end of period (in shares) at Jun. 30, 2019	41,278,570
Balance at beginning of period at Dec. 31, 2019		523,505,000	(66,000)	(453,527,000)	$ 69,912,000
Balance at beginning of period (in shares) at Dec. 31, 2019	41,485,638				41,485,638
Increase (Decrease) in Stockholders' Equity
Vesting of restricted stock units		(121,000)			$ (121,000)
Vesting of restricted stock units, shares	346,582
Fair value of warrants issued		378,000			378,000
Unrealized gain (loss) on marketable securities			60,000		60,000
Foreign currency translation adjustment			53,000		53,000
Stock-based compensation expense		3,453,000			3,453,000
Net loss				(15,586,000)	(15,586,000)
Balance at end of period at Mar. 31, 2020		527,215,000	47,000	(469,113,000)	58,149,000
Balance at end of period (in shares) at Mar. 31, 2020	41,832,220
Balance at beginning of period at Dec. 31, 2019		523,505,000	(66,000)	(453,527,000)	$ 69,912,000
Balance at beginning of period (in shares) at Dec. 31, 2019	41,485,638				41,485,638
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities					$ 23,000
Net loss					(27,183,000)
Balance at end of period at Jun. 30, 2020		530,061,000	15,000	(480,710,000)	$ 49,366,000
Balance at end of period (in shares) at Jun. 30, 2020	42,691,114				42,691,114
Balance at beginning of period at Mar. 31, 2020		527,215,000	47,000	(469,113,000)	$ 58,149,000
Balance at beginning of period (in shares) at Mar. 31, 2020	41,832,220
Increase (Decrease) in Stockholders' Equity
Vesting of restricted stock units	$ 858,894	(463,000)			(463,000)
Unrealized gain (loss) on marketable securities			(37,000)		(37,000)
Foreign currency translation adjustment			5,000		5,000
Stock-based compensation expense		3,309,000			3,309,000
Net loss				(11,597,000)	(11,597,000)
Balance at end of period at Jun. 30, 2020		$ 530,061,000	$ 15,000	$ (480,710,000)	$ 49,366,000
Balance at end of period (in shares) at Jun. 30, 2020	42,691,114				42,691,114