Condensed Consolidated Statements of Stockholders' Deficit - USD ($) shares in Thousands, $ in Thousands	Total	At-the-market Issuance	Facility Financing Obligation	Related Party Notes	Senior Convertible Notes	Common Stock	Common Stock At-the-market Issuance	Common Stock Facility Financing Obligation	Common Stock Related Party Notes	Common Stock Senior Convertible Notes	Additional Paid-in Capital	Additional Paid-in Capital At-the-market Issuance	Additional Paid-in Capital Facility Financing Obligation	Additional Paid-in Capital Related Party Notes	Additional Paid-in Capital Senior Convertible 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 shares from the release of restricted stock units	(81)					$ 1					(82)
Issuance of common shares from the release of restricted stock units (in shares)						60
Issuance of common shares under Employee Stock Purchase Plan	1					$ 1
Issuance of common shares under Employee Stock Purchase Plan (in shares)						137
Stock-based compensation expense	1,943										1,943
Issuance of shares pursuant to conversion notes			$ 9,407	$ 8,160				$ 35	$ 30				$ 9,372	$ 8,130
Issuance of shares pursuant to conversion notes (in shares)								3,549	3,000
Cumulative translation gain (loss)	3															3
Amortization of shelf fees	(5)										(5)
Issuance of at-the-market placement		$ 634					$ 2					$ 632
Issuance of at-the-market placement (in shares)							214
Issuance costs associated with at-the-market placement	(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
Net loss	(53,060)
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
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
Exercise of stock options	2										2
Exercise of stock options (in shares)						3
Issuance of common shares from the release of restricted stock units	(104)					$ 1					(105)
Issuance of common shares from the release of restricted stock units (in shares)						108
Stock-based compensation expense	2,209										2,209
Issuance of shares pursuant to conversion notes			$ 6,000		$ 4,442			$ 31		$ 22			$ 5,969		$ 4,420
Issuance of shares pursuant to conversion notes (in shares)								3,061		2,250
Cumulative translation gain (loss)	(3)															(3)
Amortization of shelf fees	(7)										(7)
Issuance of shares under Market Price StockPurchase Plan	335					$ 2					333
Issuance of shares under Market Price Stock Purchase Plan (in shares)						184
Issuance of direct placement — common stock	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
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
Exercise of stock options	3										3
Exercise of stock options (in shares)						3
Issuance of common shares from the release of restricted stock units	(1)					$ 1					(2)
Issuance of common shares from the release of restricted stock units (in shares)						63
Issuance of common shares under Employee Stock Purchase Plan	317					$ 3					314
Issuance of common shares under Employee Stock Purchase Plan (in shares)						296
Stock-based compensation expense	1,104										1,104
Issuance of shares pursuant to conversion notes					$ 538					$ 4					$ 534
Issuance of shares 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
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
Exercise of stock options	15										15
Exercise of stock options (in shares)						14
Issuance of common shares from the release of restricted stock units						$ 1					(1)
Issuance of common shares from the release of restricted stock units (in shares)						87
Stock-based compensation expense	2,568										2,568
Issuance of at-the-market placement		$ 1,850					$ 15					$ 1,835
Issuance of at-the-market placement (in shares)							1,568
Issuance costs associated with at-the-market placement	(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