CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total		Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Balances at beginning of period at Dec. 31, 2017	$ 62,783		$ 6	$ 291,163	$ (227,704)	$ (682)
Balances at beginning of period (in shares) at Dec. 31, 2017			5,729
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of adoption of ASC 606 | ASC 606 [Member]	4,235		$ 0	0	4,235	0
Issuance of common stock from vesting of restricted stock units	0		$ 0	0	0	0
Issuance of common stock from vesting of restricted stock units (in shares)			30
Tax withholding related to vesting of restricted stock units	(137)		$ 0	(137)	0	0
Issuance of common stock under employee stock purchase plan	172		$ 0	172	0	0
Issuance of common stock under employee stock purchase plan (in shares)			31
Stock-based compensation expense	2,933		$ 0	2,933	0	0
Net loss	(38,882)		0	0	(38,882)	0
Foreign currency translation adjustments	(223)		0	4	0	(227)
Balances at end of period at Sep. 30, 2018	30,881		$ 6	294,135	(262,351)	(909)
Balances at end of period (in shares) at Sep. 30, 2018			5,790
Balances at beginning of period at Dec. 31, 2017	62,783		$ 6	291,163	(227,704)	(682)
Balances at beginning of period (in shares) at Dec. 31, 2017			5,729
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(41,244)
Balances at end of period at Dec. 31, 2018	29,371	[1]	$ 6	295,116	(264,713)	(1,038)
Balances at end of period (in shares) at Dec. 31, 2018			5,938
Balances at beginning of period at Jun. 30, 2018	51,611		$ 6	293,278	(240,857)	(816)
Balances at beginning of period (in shares) at Jun. 30, 2018			5,784
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from vesting of restricted stock units	0		$ 0	0	0	0
Issuance of common stock from vesting of restricted stock units (in shares)			6
Tax withholding related to vesting of restricted stock units	(18)		$ 0	(18)	0	0
Stock-based compensation expense	875		0	875	0	0
Net loss	(21,494)		0	0	(21,494)	0
Foreign currency translation adjustments	(93)		0	0	0	(93)
Balances at end of period at Sep. 30, 2018	30,881		$ 6	294,135	(262,351)	(909)
Balances at end of period (in shares) at Sep. 30, 2018			5,790
Balances at beginning of period at Dec. 31, 2018	29,371	[1]	$ 6	295,116	(264,713)	(1,038)
Balances at beginning of period (in shares) at Dec. 31, 2018			5,938
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through at-the-market offering, net of offering costs	$ 1,504		$ 1	1,503	0	0
Issuance of common stock through at-the-market offering, net of offering costs (in shares)	570		570
Issuance of common stock from vesting of restricted stock units	$ 0		$ 0	0	0	0
Issuance of common stock from vesting of restricted stock units (in shares)			92
Tax withholding related to vesting of restricted stock units	(254)		$ 0	(254)	0	0
Issuance of common stock under employee stock purchase plan	88		$ 0	88	0	0
Issuance of common stock under employee stock purchase plan (in shares)			42
Stock-based compensation expense	2,100		$ 0	2,100	0	0
Net loss	(11,882)		0	0	(11,882)	0
Foreign currency translation adjustments	(49)		0	0	0	(49)
Balances at end of period at Sep. 30, 2019	20,878		$ 7	298,553	(276,595)	(1,087)
Balances at end of period (in shares) at Sep. 30, 2019			6,642
Balances at beginning of period at Jun. 30, 2019	$ 23,555		$ 7	297,903	(273,322)	(1,033)
Balances at beginning of period (in shares) at Jun. 30, 2019			6,623
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through at-the-market offering, net of offering costs (in shares)	0
Issuance of common stock from vesting of restricted stock units	$ 0		$ 0	0	0	0
Issuance of common stock from vesting of restricted stock units (in shares)			19
Tax withholding related to vesting of restricted stock units	(3)		$ 0	(3)	0	0
Stock-based compensation expense	653		0	653	0	0
Net loss	(3,273)		0	0	(3,273)	0
Foreign currency translation adjustments	(54)		0	0	0	(54)
Balances at end of period at Sep. 30, 2019	$ 20,878		$ 7	$ 298,553	$ (276,595)	$ (1,087)
Balances at end of period (in shares) at Sep. 30, 2019			6,642

[1] Derived from the Company’s audited consolidated financial statements as of December 31, 2018.