Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Total	At-the-market Offering	Facility Financing Obligation	Related Party Notes	Senior Convertible Notes	Promissory Notes	Common Stock	Common Stock At-the-market Offering	Common Stock Facility Financing Obligation	Common Stock Related Party Notes	Common Stock Senior Convertible Notes	Common Stock Promissory Notes	Additional Paid-in Capital	Additional Paid-in Capital At-the-market Offering	Additional Paid-in Capital Facility Financing Obligation	Additional Paid-in Capital Related Party Notes	Additional Paid-in Capital Senior Convertible Notes	Additional Paid-in Capital Promissory Notes	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2017	$ (214,733)						$ 1,191						$ 2,638,992						$ (18)	$ (2,854,898)
Beginning Balance (in shares) at Dec. 31, 2017							119,053
Adjustment to adopt ASU 2016-09 | Topic 718	1,873																			1,873
Issuance of common stock from the release of restricted stock units	(81)						$ 1						(82)
Issuance of common stock from the release of restricted stock units (in shares)							60
Issuance of common stock under Employee Stock Purchase Plan	1						$ 1
Issuance of common stock under Employee Stock Purchase Plan (in shares)							137
Stock-based compensation expense	1,943												1,943
Issuance of common stock pursuant to conversion notes			$ 9,407	$ 8,160					$ 35	$ 30					$ 9,372	$ 8,130
Issuance of common stock pursuant to conversion notes (in shares)									3,549	3,000
Cumulative translation gain (loss)	3																		3
Amortization of shelf fees	(5)												(5)
Issuance of common stock in at-the-market offering		$ 634						$ 2						$ 632
Issuance of common stock in at-the-market offering (in shares)								214
Issuance costs associated with at-the-market offering	(25)												(25)
Net loss	(30,385)																			(30,385)
Ending Balance at Mar. 31, 2018	(223,208)						$ 1,260						2,658,957						(15)	(2,883,410)
Ending Balance (in shares) at Mar. 31, 2018							126,013
Beginning Balance at Dec. 31, 2017	(214,733)						$ 1,191						2,638,992						(18)	(2,854,898)
Beginning Balance (in shares) at Dec. 31, 2017							119,053
Cumulative translation gain (loss)	(1)
Ending Balance at Sep. 30, 2018	(205,441)						$ 1,594						2,723,232						(18)	(2,930,249)
Ending Balance (in shares) at Sep. 30, 2018							159,497
Beginning Balance at Mar. 31, 2018	(223,208)						$ 1,260						2,658,957						(15)	(2,883,410)
Beginning Balance (in shares) at Mar. 31, 2018							126,013
Issuance of common stock from the release of restricted stock units	(104)						$ 1						(105)
Issuance of common stock from the release of restricted stock units (in shares)							108
Exercise of stock options	2												2
Exercise of stock options (in shares)							3
Stock-based compensation expense	2,209												2,209
Issuance of common stock pursuant to conversion notes			6,000		$ 4,442				$ 31		$ 22				5,969		$ 4,420
Issuance of common stock pursuant to conversion notes (in shares)									3,061		2,250
Cumulative translation gain (loss)	(3)																		(3)
Amortization of shelf fees	(7)												(7)
Issuance of common stock under Market Price Stock Purchase Plan	335						$ 2						333
Issuance of shares under Market Price Stock Purchase Plan (in shares)							184
Issuance of common stock in direct placement	27,860												27,860
Issuance costs associated with direct placement	(1,470)						$ 140						(1,610)
Issuance costs associated with direct placement (in shares)							14,000
Net loss	(22,675)																			(22,675)
Ending Balance at Jun. 30, 2018	(206,619)						$ 1,456						2,698,028						(18)	(2,906,085)
Ending Balance (in shares) at Jun. 30, 2018							145,619
Issuance of common stock from the release of restricted stock units (in shares)							20
Exercise of stock options	2												2
Exercise of stock options (in shares)							2
Issuance of common stock under Employee Stock Purchase Plan	343						$ 2						341
Issuance of common stock under Employee Stock Purchase Plan (in shares)							263
Stock-based compensation expense	1,562												1,562
Issuance of common stock pursuant to conversion notes			22,721		538				$ 131		$ 5				22,590		533
Issuance of common stock pursuant to conversion notes (in shares)									13,117		476
Issuance of common stock in at-the-market offering		183												183
Issuance costs associated with at-the-market offering	(7)												(7)
Net loss	(24,164)																			(24,164)
Ending Balance at Sep. 30, 2018	(205,441)						$ 1,594						2,723,232						(18)	(2,930,249)
Ending Balance (in shares) at Sep. 30, 2018							159,497
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
Issuance of common stock from the release of restricted stock units	(1)						$ 1						(2)
Issuance of common stock from the release of restricted stock units (in shares)							63
Exercise of stock options	3												3
Exercise of stock options (in shares)							3
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	1,104												1,104
Issuance of common stock pursuant to conversion notes					538						$ 4						534
Issuance of common stock pursuant to conversion notes (in shares)											386
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
Cumulative translation gain (loss)	(1)
Ending Balance at Sep. 30, 2019	(184,706)						$ 2,064						2,790,890						(20)	(2,977,640)
Ending Balance (in shares) at Sep. 30, 2019							206,408
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
Issuance of common stock from the release of restricted stock units							$ 1						(1)
Issuance of common stock from the release of restricted stock units (in shares)							87
Exercise of stock options	15												15
Exercise of stock options (in shares)							14
Stock-based compensation expense	2,568												2,568
Issuance of common stock in at-the-market offering		1,850						$ 15						1,835
Issuance of common stock in at-the-market offering (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
Issuance of common stock from the release of restricted stock units							$ 6						(6)
Issuance of common stock from the release of restricted stock units (in shares)							547
Exercise of stock options	31												31
Exercise of stock options (in shares)							29
Issuance of common stock under Employee Stock Purchase Plan	339						$ 4						335
Issuance of common stock under Employee Stock Purchase Plan (in shares)							356
Stock-based compensation expense	1,640												1,640
Issuance of common stock pursuant to conversion notes			$ 4,575		$ 5,037	$ 8,000			$ 42		$ 45	$ 71			$ 4,533		$ 4,992	$ 7,929
Issuance of common stock pursuant to conversion notes (in shares)									4,193		4,525	7,143
Cumulative translation gain (loss)	(1)
Issuance of common stock in at-the-market offering		$ 188						$ 2						$ 186
Issuance of common stock in at-the-market offering (in shares)								168
Issuance of warrants pursuant to Credit facility	1,854												1,854
Net loss	(10,371)																		(1)	(10,370)
Ending Balance at Sep. 30, 2019	$ (184,706)						$ 2,064						$ 2,790,890						$ (20)	$ (2,977,640)
Ending Balance (in shares) at Sep. 30, 2019							206,408