CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]	Total
Balances at beginning of period at Dec. 31, 2017	$ 6	$ 291,163	$ (227,704)	$ (682)	$ 62,783
Balances at beginning of period (in shares) at Dec. 31, 2017	5,729
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)	24
Issuance of common stock under employee stock purchase plan	$ 0	172	0	0	172
Issuance of common stock under employee stock purchase plan (in shares)	31
Tax withholding related to vesting of restricted stock units	$ 0	(119)	0	0	(119)
Stock-based compensation expense	0	2,058	0	0	2,058
Net loss	0	0	(17,388)	0	(17,388)
Foreign currency translation adjustments	0	4	0	(134)	(130)
Balances at end of period at Jun. 30, 2018	$ 6	293,278	(240,857)	(816)	51,611
Balances at end of period (in shares) at Jun. 30, 2018	5,784
Balances at beginning of period at Dec. 31, 2017	$ 6	291,163	(227,704)	(682)	62,783
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	$ 6	295,116	(264,713)	(1,038)	29,371	[1]
Balances at end of period (in shares) at Dec. 31, 2018	5,938
Balances at beginning of period at Mar. 31, 2018	$ 6	292,099	(232,581)	(238)	59,286
Balances at beginning of period (in shares) at Mar. 31, 2018	5,748
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)	5
Issuance of common stock under employee stock purchase plan	$ 0	172	0	0	172
Issuance of common stock under employee stock purchase plan (in shares)	31
Tax withholding related to vesting of restricted stock units	$ 0	(23)	0	0	(23)
Stock-based compensation expense	0	1,030	0	0	1,030
Net loss	0	0	(8,276)	0	(8,276)
Foreign currency translation adjustments	0	0	0	(578)	(578)
Balances at end of period at Jun. 30, 2018	$ 6	293,278	(240,857)	(816)	51,611
Balances at end of period (in shares) at Jun. 30, 2018	5,784
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of adoption of ASC 606 | ASC 606 [Member]	$ 0	0	4,235	0	4,235
Balances at beginning of period at Dec. 31, 2018	$ 6	295,116	(264,713)	(1,038)	29,371	[1]
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 equity distribution agreement, net of offering costs	$ 1	1,503	0	0	$ 1,504
Issuance of common stock through equity distribution agreement, 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)	73
Issuance of common stock under employee stock purchase plan	$ 0	88	0	0	88
Issuance of common stock under employee stock purchase plan (in shares)	42
Tax withholding related to vesting of restricted stock units	$ 0	(251)	0	0	(251)
Stock-based compensation expense	0	1,447	0	0	1,447
Net loss	0	0	(8,609)	0	(8,609)
Foreign currency translation adjustments	0	0	0	5	5
Balances at end of period at Jun. 30, 2019	$ 7	297,903	(273,322)	(1,033)	23,555
Balances at end of period (in shares) at Jun. 30, 2019	6,623
Balances at beginning of period at Mar. 31, 2019	$ 6	295,745	(269,319)	(1,109)	25,323
Balances at beginning of period (in shares) at Mar. 31, 2019	5,954
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through equity distribution agreement, net of offering costs	$ 1	1,503	0	0	$ 1,504
Issuance of common stock through equity distribution agreement, 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)	57
Issuance of common stock under employee stock purchase plan	$ 0	88	0	0	88
Issuance of common stock under employee stock purchase plan (in shares)	42
Tax withholding related to vesting of restricted stock units	$ 0	(195)	0	0	(195)
Stock-based compensation expense	0	762	0	0	762
Net loss	0	0	(4,003)	0	(4,003)
Foreign currency translation adjustments	0	0	0	76	76
Balances at end of period at Jun. 30, 2019	$ 7	$ 297,903	$ (273,322)	$ (1,033)	$ 23,555
Balances at end of period (in shares) at Jun. 30, 2019	6,623

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