Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Follow-On Public Offering [Member]	"At-The-Market" Equity Offering Sales Agreement [Member]	Common Shares [Member]	Common Shares [Member] Follow-On Public Offering [Member]	Common Shares [Member] "At-The-Market" Equity Offering Sales Agreement [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Follow-On Public Offering [Member]	Additional Paid-In Capital [Member] "At-The-Market" Equity Offering Sales Agreement [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2014	$ 108,566			$ 31			$ 294,998			$ (186,463)
Beginning Balance, shares at Dec. 31, 2014				30,806,000
Issuance of common stock under stock plans	4,672			$ 1			4,671
Issuance of common stock under stock plans, shares				818,000
Issuance of common stock from offering		$ 162,151			$ 5			$ 162,146
Issuance of common stock from offering, shares					4,945,000
Shares issued in connection with employee stock purchase plan	239						239
Shares issued in connection with employee stock purchase plan, shares				16,000
Stock-based compensation expense	11,616						11,616
Net loss	(83,189)									(83,189)
Ending Balance at Dec. 31, 2015	204,055			$ 37			473,670			(269,652)
Ending Balance, shares at Dec. 31, 2015				36,585,000
Issuance of common stock under stock plans	146						146
Issuance of common stock under stock plans, shares				298,000
Shares issued in connection with employee stock purchase plan	187						187
Shares issued in connection with employee stock purchase plan, shares				59,000
Stock-based compensation expense	13,145						13,145
Net loss	(77,480)									(77,480)
Ending Balance at Dec. 31, 2016	140,053			$ 37			487,148			(347,132)
Ending Balance, shares at Dec. 31, 2016				36,942,000
Issuance of common stock under stock plans	$ 289						289
Issuance of common stock under stock plans, shares	88,319			173,000
Issuance of common stock from offering		$ 61,394	$ 31,142		$ 10	$ 4		$ 61,384	$ 31,138
Issuance of common stock from offering, shares					10,107,000	4,158,000
Shares issued in connection with employee stock purchase plan	$ 321						321
Shares issued in connection with employee stock purchase plan, shares				78,000
Stock-based compensation expense	11,963						11,963
Net loss	(114,752)									(114,752)
Ending Balance at Dec. 31, 2017	$ 130,410			$ 51			$ 592,243			$ (461,884)
Ending Balance, shares at Dec. 31, 2017				51,458,000