Condensed Consolidated Statements of Stockholders’ Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional paid-in capital	Treasury stock	Accumulated other comprehensive income (loss)	Accumulated deficit	Total
Beginning balance at Dec. 31, 2017	$ 2	$ 523,012	$ (35,706)	$ (17)	$ (428,771)	$ 58,520
Beginning balance (in shares) at Dec. 31, 2017	21,031
Issuance of stock under employee stock awards and other, net		148				148
Issuance of stock under employee stock awards and other, net (in shares)	326
Tax withholdings related to vesting of share-based payments		(1,402)				(1,402)
Stock-based compensation expense		2,320				2,320
Issuance of common stock in connection with follow-on public offering, net of offering costs		23,367				23,367
Issuance of common stock in connection with follow-on public offering, net of offering costs (in shares)	3,373
Foreign currency translation adjustment				52		52
Net loss					(5,925)	(5,925)
Ending balance at Mar. 31, 2018	$ 2	547,445	(35,706)	35	(434,696)	77,080
Ending balance (in shares) at Mar. 31, 2018	24,730
Beginning balance at Dec. 31, 2017	$ 2	523,012	(35,706)	(17)	(428,771)	58,520
Beginning balance (in shares) at Dec. 31, 2017	21,031
Foreign currency translation adjustment						1
Net loss						(18,253)
Ending balance at Sep. 30, 2018	$ 2	552,452	(35,706)	(16)	(447,024)	69,708
Ending balance (in shares) at Sep. 30, 2018	25,265
Beginning balance at Mar. 31, 2018	$ 2	547,445	(35,706)	35	(434,696)	77,080
Beginning balance (in shares) at Mar. 31, 2018	24,730
Issuance of stock under employee stock awards and other, net		481				481
Issuance of stock under employee stock awards and other, net (in shares)	250
Tax withholdings related to vesting of share-based payments		(866)				(866)
Stock-based compensation expense		2,835				2,835
Foreign currency translation adjustment				(80)		(80)
Net loss					(6,293)	(6,293)
Ending balance at Jun. 30, 2018	$ 2	549,895	(35,706)	(45)	(440,989)	73,157
Ending balance (in shares) at Jun. 30, 2018	24,980
Issuance of stock under employee stock awards and other, net		840				840
Issuance of stock under employee stock awards and other, net (in shares)	285
Tax withholdings related to vesting of share-based payments		(915)				(915)
Stock-based compensation expense		2,632				2,632
Foreign currency translation adjustment				29		29
Net loss					(6,035)	(6,035)
Ending balance at Sep. 30, 2018	$ 2	552,452	(35,706)	(16)	(447,024)	69,708
Ending balance (in shares) at Sep. 30, 2018	25,265
Beginning balance at Dec. 31, 2018	$ 3	554,403	(35,706)	(52)	(451,961)	$ 66,687
Beginning balance (in shares) at Dec. 31, 2018	25,483					25,483
Issuance of stock under employee stock awards and other, net		270				$ 270
Issuance of stock under employee stock awards and other, net (in shares)	319
Tax withholdings related to vesting of share-based payments		(1,322)				(1,322)
Stock-based compensation expense		2,045				2,045
Foreign currency translation adjustment				27		27
Net loss					(10,286)	(10,286)
Ending balance at Mar. 31, 2019	$ 3	555,396	(35,706)	(25)	(462,247)	57,421
Ending balance (in shares) at Mar. 31, 2019	25,802
Beginning balance at Dec. 31, 2018	$ 3	554,403	(35,706)	(52)	(451,961)	$ 66,687
Beginning balance (in shares) at Dec. 31, 2018	25,483					25,483
Foreign currency translation adjustment						$ (37)
Net loss						(21,533)
Ending balance at Sep. 30, 2019	$ 3	559,664	(35,706)	(89)	(473,494)	$ 50,378
Ending balance (in shares) at Sep. 30, 2019	26,150					26,150
Beginning balance at Mar. 31, 2019	$ 3	555,396	(35,706)	(25)	(462,247)	$ 57,421
Beginning balance (in shares) at Mar. 31, 2019	25,802
Issuance of stock under employee stock awards and other, net		175				175
Issuance of stock under employee stock awards and other, net (in shares)	187
Tax withholdings related to vesting of share-based payments		(717)				(717)
Stock-based compensation expense		2,331				2,331
Foreign currency translation adjustment				(24)		(24)
Net loss					(6,762)	(6,762)
Ending balance at Jun. 30, 2019	$ 3	557,185	(35,706)	(49)	(469,009)	52,424
Ending balance (in shares) at Jun. 30, 2019	25,989
Issuance of stock under employee stock awards and other, net		281				281
Issuance of stock under employee stock awards and other, net (in shares)	161
Tax withholdings related to vesting of share-based payments		(364)				(364)
Stock-based compensation expense		2,562				2,562
Foreign currency translation adjustment				(40)		(40)
Net loss					(4,485)	(4,485)
Ending balance at Sep. 30, 2019	$ 3	$ 559,664	$ (35,706)	$ (89)	$ (473,494)	$ 50,378
Ending balance (in shares) at Sep. 30, 2019	26,150					26,150