Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning Balance, (shares) at Dec. 31, 2018		110,525,141
Beginning Balance at Dec. 31, 2018	$ 271,410	$ 110	$ 886,740	$ (3,702)	$ (611,738)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(19,259)				(19,259)
Other comprehensive loss, net	(390)			(390)
Recognition of stock-based compensation expense	7,481		7,481
Issuance of common stock to 401(k) plan for employees (shares)		47,347
Issuance of common stock to 401(k) plan for employees	519		519
Issuance of common stock upon exercise of stock options (shares)		146,785
Issuance of common stock upon exercise of stock options	831			831
Restricted stock award grants, net of forfeitures (shares)		259,539
Restricted stock award grants, net of forfeitures	1	$ 1
Issuance of common stock upon vesting of restricted stock units (shares)		233,760
Issuance of common stock upon vesting of restricted stock units	0
Ending Balance, (shares) at Mar. 31, 2019		111,212,572
Ending Balance at Mar. 31, 2019	260,593	$ 111	895,571	(4,092)	(630,997)
Beginning Balance, (shares) at Dec. 31, 2018		110,525,141
Beginning Balance at Dec. 31, 2018	271,410	$ 110	886,740	(3,702)	(611,738)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(47,654)
Ending Balance, (shares) at Jun. 30, 2019		112,684,387
Ending Balance at Jun. 30, 2019	242,827	$ 112	905,871	(3,764)	(659,392)
Beginning Balance, (shares) at Mar. 31, 2019		111,212,572
Beginning Balance at Mar. 31, 2019	260,593	$ 111	895,571	(4,092)	(630,997)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(28,395)				(28,395)
Other comprehensive loss, net	328			328
Recognition of stock-based compensation expense	4,814		4,814
Issuance of common stock to 401(k) plan for employees (shares)		24,382
Issuance of common stock to 401(k) plan for employees	205		205
Issuance of common stock for employee stock purchase plan (shares)		60,606
Issuance of common stock for employee stock purchase plan	444		444
Issuance of common stock upon exercise of stock options (shares)		504,226
Issuance of common stock upon exercise of stock options	3,023		3,023
Restricted stock award grants, net of forfeitures (shares)		651,072
Restricted stock award grants, net of forfeitures	1	$ 1
Issuance of common stock upon vesting of restricted stock units (shares)		10,000
Issuance of common stock upon vesting of restricted stock units	$ 0
Issuance of common shares under an at-the-market sales agreement (Note 12) (shares)	221,529	221,529
Issuance of common shares under an at-the-market sales agreement (Note 12)	$ 1,814		1,814
Ending Balance, (shares) at Jun. 30, 2019		112,684,387
Ending Balance at Jun. 30, 2019	242,827	$ 112	905,871	(3,764)	(659,392)
Beginning Balance, (shares) at Dec. 31, 2019		113,299,612
Beginning Balance at Dec. 31, 2019	190,393	$ 113	918,205	(3,498)	(724,427)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(40,572)				(40,572)
Other comprehensive loss, net	(1,329)			(1,329)
Recognition of stock-based compensation expense	5,010		5,010
Issuance of common stock to 401(k) plan for employees (shares)		96,959
Issuance of common stock to 401(k) plan for employees	265		265
Restricted stock award grants, net of forfeitures (shares)		1,377,508
Restricted stock award grants, net of forfeitures	1	$ 1
Ending Balance, (shares) at Mar. 31, 2020		114,774,079
Ending Balance at Mar. 31, 2020	153,768	$ 114	923,480	(4,827)	(764,999)
Beginning Balance, (shares) at Dec. 31, 2019		113,299,612
Beginning Balance at Dec. 31, 2019	190,393	$ 113	918,205	(3,498)	(724,427)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(72,657)
Ending Balance, (shares) at Jun. 30, 2020		117,823,973
Ending Balance at Jun. 30, 2020	130,597	$ 118	930,817	(3,254)	(797,084)
Beginning Balance, (shares) at Mar. 31, 2020		114,774,079
Beginning Balance at Mar. 31, 2020	153,768	$ 114	923,480	(4,827)	(764,999)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(32,085)				(32,085)
Other comprehensive loss, net	1,573			1,573
Recognition of stock-based compensation expense	3,988		3,988
Issuance of common stock for employee stock purchase plan (shares)		98,362
Issuance of common stock for employee stock purchase plan	282		282
Restricted stock award grants, net of forfeitures (shares)		1,926,385
Restricted stock award grants, net of forfeitures	0	$ 3	(3)
Issuance of common stock upon vesting of restricted stock units (shares)		861
Issuance of common stock upon vesting of restricted stock units	$ 0
Issuance of common shares under an at-the-market sales agreement (Note 12) (shares)	1,024,286	1,024,286
Issuance of common shares under an at-the-market sales agreement (Note 12)	$ 3,071	$ 1	3,070
Ending Balance, (shares) at Jun. 30, 2020		117,823,973
Ending Balance at Jun. 30, 2020	$ 130,597	$ 118	$ 930,817	$ (3,254)	$ (797,084)