Condensed Consolidated Statements of Changes in Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 177	$ 428,878	$ (357,794)	$ 71,261
Balance (in shares) at Dec. 31, 2018	176,918,000
Changes in Stockholders' Equity (Deficit)
Issuance of common stock, net	$ 26	26,731		26,757
Issuance of common stock (in shares)	26,136,000
Exercise of stock options		93		93
Exercise of stock options (in shares)	230,000
Stock-based compensation expense and vesting of RSUs		6,451		6,451
Stock-based compensation expense and vesting of RSUs (in shares)	82,000
Issuance of warrants related to debt, net		723		723
Net loss			(79,938)	(79,938)
Balance at Sep. 30, 2019	$ 203	462,876	(437,732)	25,347
Balance (in shares) at Sep. 30, 2019	203,366,000
Balance at Jun. 30, 2019	$ 177	433,228	(418,233)	15,172
Balance (in shares) at Jun. 30, 2019	177,031,000
Changes in Stockholders' Equity (Deficit)
Issuance of common stock, net	$ 26	26,731		26,757
Issuance of common stock (in shares)	26,136,000
Exercise of stock options (in shares)	179,000
Stock-based compensation expense and vesting of RSUs		2,194		2,194
Stock-based compensation expense and vesting of RSUs (in shares)	20,000
Issuance of warrants related to debt, net		723		723
Net loss			(19,499)	(19,499)
Balance at Sep. 30, 2019	$ 203	462,876	(437,732)	25,347
Balance (in shares) at Sep. 30, 2019	203,366,000
Balance at Dec. 31, 2019	$ 203	464,491	(473,343)	(8,649)
Balance (in shares) at Dec. 31, 2019	203,453,000
Changes in Stockholders' Equity (Deficit)
Issuance of common stock, net		(86)		$ (86)
Issuance of common stock (in shares)	175,000			175,289
Exercise of stock options and ESPP purchases	$ 2	573		$ 575
Exercise of stock options and ESPP purchases (in shares)	2,220,000
Exchange and conversion of convertible notes, net	$ 39	20,082		20,121
Exchange and conversion of convertible notes, net (in shares)	38,391,000
Stock-based compensation expense and vesting of RSUs		5,581		5,581
Issuance of warrants related to debt, net		1,212		1,212
Net loss			(73,536)	(73,536)
Balance at Sep. 30, 2020	$ 244	491,853	(546,879)	(54,782)
Balance (in shares) at Sep. 30, 2020	244,239,000
Balance at Jun. 30, 2020	$ 231	483,615	(523,451)	$ (39,605)
Balance (in shares) at Jun. 30, 2020	230,553,000
Changes in Stockholders' Equity (Deficit)
Issuance of common stock (in shares)				0
Exercise of stock options and ESPP purchases	$ 1	76		$ 77
Exercise of stock options and ESPP purchases (in shares)	1,188,000
Exchange and conversion of convertible notes, net	$ 12	5,859		5,871
Exchange and conversion of convertible notes, net (in shares)	12,498,000
Stock-based compensation expense and vesting of RSUs		2,303		2,303
Net loss			(23,428)	(23,428)
Balance at Sep. 30, 2020	$ 244	$ 491,853	$ (546,879)	$ (54,782)
Balance (in shares) at Sep. 30, 2020	244,239,000