Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Total	At-the-market Offering	June 2020 Note	Senior Convertible Notes	Common Stock	Common Stock At-the-market Offering	Common Stock June 2020 Note	Common Stock Senior Convertible Notes	Additional Paid-in Capital	Additional Paid-in Capital At-the-market Offering	Additional Paid-in Capital June 2020 Note	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, 2018	(175,082)				$ 1,870				2,763,067				(19)	(2,940,000)
Beginning Balance (in shares) at Dec. 31, 2018					187,030
Net loss	(27,270)
Ending Balance at Jun. 30, 2019	(195,999)				$ 1,894				2,769,396				(19)	(2,967,270)
Ending Balance (in shares) at Jun. 30, 2019					189,447
Beginning Balance at Mar. 31, 2019	(188,004)				$ 1,878				2,765,020				(19)	(2,954,883)
Beginning Balance (in shares) at Mar. 31, 2019					187,778
Net issuance of common stock associated with stock options and restricted stock units	15				$ 1				14
Net issuance of common stock associated with stock options and restricted stock units (in shares)					101
Stock-based compensation expense	2,568								2,568
Issuance of common stock		$ 1,850				$ 15				$ 1,835
Issuance of common stock (in shares)						1,568
Issuance costs associated with at-the-market offering	(41)								(41)
Net loss	(12,387)													(12,387)
Ending Balance at Jun. 30, 2019	(195,999)				$ 1,894				2,769,396				(19)	(2,967,270)
Ending Balance (in shares) at Jun. 30, 2019					189,447
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 costs 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
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
Cumulative translation loss	19
Net loss	(19,574)
Ending Balance at Jun. 30, 2020	(179,710)				$ 2,289				2,829,478					(3,011,477)
Ending Balance (in shares) at Jun. 30, 2020					228,928
Beginning Balance at Mar. 31, 2020	(198,050)				$ 2,131				2,801,044					(3,001,225)
Beginning Balance (in shares) at Mar. 31, 2020					213,138
Net issuance of common stock associated with stock options and restricted stock units	117				$ 3				114
Net issuance of common stock associated with stock options and restricted stock units (in shares)					297
Stock-based compensation expense	2,185								2,185
Issuance of common stock from the exercise of warrants	11,600				$ 73				11,527
Issuance of common stock from the exercise of warrants (in Share)					7,250
Issuance of common stock pursuant to conversion notes			$ 2,630				$ 12				$ 2,618
Issuance of common stock pursuant to conversion notes (in shares)							1,235
Issuance of common stock		$ 12,366				$ 75				$ 12,291
Issuance of common stock (in shares)						7,459
Issuance costs associated with at-the-market offering	(320)								(320)
Issuance of common stock from market price stock purchase	14								14
Issuance of common stock under Market Price Stock Purchase Plan (in shares)					10
Adjustment of common stock in association with restricted stock units					$ (5)				5
Adjustment of common stock in association with restricted stock units (in shares)					(461)
Net loss	(10,252)													(10,252)
Ending Balance at Jun. 30, 2020	$ (179,710)				$ 2,289				$ 2,829,478					$ (3,011,477)
Ending Balance (in shares) at Jun. 30, 2020					228,928