Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Total	At-the-market Offering	Senior Convertible Notes	Common Stock	Common Stock At-the-market Offering	Common Stock Senior Convertible Notes	Additional Paid-in Capital	Additional Paid-in Capital At-the-market Offering	Additional Paid-in Capital Senior Convertible Notes	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ (175,082)			$ 1,870			$ 2,763,067			$ (19)	$ (2,940,000)
Beginning Balance (in shares) at Dec. 31, 2018				187,030
Net issuance of common stock associated with stock options and restricted stock units	2			$ 1			1
Net issuance of common stock associated with stock options and restricted stock units (in shares)				66
Issuance of common stock under Employee Stock Purchase Plan	317			$ 3			314
Issuance of common stock under Employee Stock Purchase Plan (in shares)				296
Stock-based compensation expense	999						999
Issuance of common stock pursuant to conversion notes			$ 538			$ 4			$ 534
Issuance of common stock pursuant to conversion notes (in shares)						386
Restricted stock unit award	105						105
Net loss	(14,883)										(14,883)
Ending Balance at Mar. 31, 2019	(188,004)			$ 1,878			2,765,020			(19)	(2,954,883)
Ending Balance (in shares) at Mar. 31, 2019				187,778
Beginning Balance at Dec. 31, 2019	(190,526)			$ 2,118			2,799,278			(19)	(2,991,903)
Beginning Balance (in shares) at Dec. 31, 2019				211,788
Net issuance of common stock associated with stock options and restricted stock units	(317)			$ 5			(322)
Net issuance of common stock associated with stock options and restricted stock units (in shares)				504
Issuance of common stock under Employee Stock Purchase Plan	318			$ 3			315
Issuance of common stock under Employee Stock Purchase Plan (in shares)				334
Stock-based compensation expense	1,128						1,128
Issuance of common stock associated with debt interest payment	144			$ 1			143
Issuance of common stock associated with debt interest payment (in shares)				99
Issuance of common stock		$ 522			$ 4			$ 518
Issuance of common stock (in shares)					413
Issuance cost associated with at-the-market offering	(16)						(16)
Cumulative translation loss	19									$ 19
Net loss	(9,322)										(9,322)
Ending Balance at Mar. 31, 2020	$ (198,050)			$ 2,131			$ 2,801,044				$ (3,001,225)
Ending Balance (in shares) at Mar. 31, 2020				213,138