CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (UNAUDITED) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	$ (39,454)			$ 800	$ (73)	$ (40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Vesting of common stock from option exercises	16			16
Vesting of common stock from option exercises, Shares			34,137
Issuance of common stock from option exercises	69			69
Issuance of common stock from option exercises, Shares			26,204
Stock-based compensation	380			380
Other comprehensive (loss) gain	(13)				(13)
Net loss	(10,217)					(10,217)
Ending balance at Mar. 31, 2018	(49,219)			1,265	(86)	(50,398)
Ending balance, Shares at Mar. 31, 2018		127,199,705
Ending balance at Mar. 31, 2018		$ 137,762
Ending balance, Shares at Mar. 31, 2018			2,697,352
Balance balance at Dec. 31, 2017		$ 137,762
Beginning balance at Dec. 31, 2017	(39,454)			800	(73)	(40,181)
Beginning balance, Shares at Dec. 31, 2017		127,199,705
Beginning balance, Shares at Dec. 31, 2017			2,637,011
Net loss	(55,577)
Ending balance at Dec. 31, 2018	196,355		$ 3	292,187	(77)	(95,758)
Ending balance, Shares at Dec. 31, 2018			37,358,526
Vesting of common stock from option exercises	18			18
Vesting of common stock from option exercises, Shares			31,584
Issuance of common stock from option exercises	$ 21			21
Issuance of common stock from option exercises, Shares	18,876		18,876
Issuance of common stock pursuant toemployee stock purchase plan	$ 396			396
Issuance of common stock pursuant to employee stock purchase plan, Shares			28,311
Stock-based compensation	1,243			1,243
Other comprehensive (loss) gain	83				83
Net loss	(23,852)					(23,852)
Ending balance at Mar. 31, 2019	$ 174,264		$ 3	$ 293,865	$ 6	$ (119,610)
Ending balance, Shares at Mar. 31, 2019			37,437,297