Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	At the Market Offering [Member] Common Stock [Member]	At the Market Offering [Member] Additional Paid-in Capital [Member]	At the Market Offering [Member]	Underwritten Public Equity Offering [Member] Common Stock [Member]	Underwritten Public Equity Offering [Member] Additional Paid-in Capital [Member]	Underwritten Public Equity Offering [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2017							$ 116	$ 965,752	$ (32)	$ (879,733)	$ 86,103
Balance, shares at Dec. 31, 2017							116,277
Net loss										(71,763)	(71,763)
Other comprehensive income (loss)									8		8
ASC606 adoption effect										189	189
Issuance of common stock in conjunction with equity plans							$ 3	4,178			4,181
Issuance of common stock in conjunction with equity plans, shares							2,007
Issuance of common stock from equity offering, net of issuance costs	$ 14	$ 32,849	$ 32,863	$ 16	$ 64,652	$ 64,668
Issuance of common stock from equity offering, net of issuance costs, shares	14,375			16,235
Stock-based compensation expense								15,631			15,631
Balance at Sep. 30, 2018							$ 149	1,083,062	(24)	(951,307)	131,880
Balance, shares at Sep. 30, 2018							148,894
Balance at Dec. 31, 2017							$ 116	965,752	(32)	(879,733)	86,103
Balance, shares at Dec. 31, 2017							116,277
Issuance of common stock from equity offering, net of issuance costs, shares						30,600
Balance at Dec. 31, 2018							$ 150	1,096,053	(36)	(982,106)	$ 114,061
Balance, shares at Dec. 31, 2018							150,244				150,244
Balance at Jun. 30, 2018							$ 132	1,011,621	(16)	(926,263)	$ 85,474
Balance, shares at Jun. 30, 2018							131,888
Net loss										(25,044)	(25,044)
Other comprehensive income (loss)									(8)		(8)
Issuance of common stock in conjunction with equity plans							$ 1	1,663			1,664
Issuance of common stock in conjunction with equity plans, shares							771
Issuance of common stock from equity offering, net of issuance costs				$ 16	$ 64,652	$ 64,668
Issuance of common stock from equity offering, net of issuance costs, shares				16,235
Stock-based compensation expense								5,126			5,126
Balance at Sep. 30, 2018							$ 149	1,083,062	(24)	(951,307)	131,880
Balance, shares at Sep. 30, 2018							148,894
Balance at Dec. 31, 2018							$ 150	1,096,053	(36)	(982,106)	$ 114,061
Balance, shares at Dec. 31, 2018							150,244				150,244
Net loss										(84,043)	$ (84,043)
Other comprehensive income (loss)									47		47
Issuance of common stock in conjunction with equity plans							$ 3	8,414			8,417
Issuance of common stock in conjunction with equity plans, shares							2,787
Stock-based compensation expense								12,532			12,532
Balance at Sep. 30, 2019							$ 153	1,116,999	11	(1,066,149)	$ 51,014
Balance, shares at Sep. 30, 2019							153,031				153,031
Balance at Jun. 30, 2019							$ 153	1,112,610	17	(1,037,026)	$ 75,754
Balance, shares at Jun. 30, 2019							152,959
Net loss										(29,123)	(29,123)
Other comprehensive income (loss)									(6)		(6)
Issuance of common stock in conjunction with equity plans								327			327
Issuance of common stock in conjunction with equity plans, shares							72
Stock-based compensation expense								4,062			4,062
Balance at Sep. 30, 2019							$ 153	$ 1,116,999	$ 11	$ (1,066,149)	$ 51,014
Balance, shares at Sep. 30, 2019							153,031				153,031