Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2014	$ 144,941	$ 21	$ 238,402	$ (5)	$ (93,477)
Balance (in shares) at Dec. 31, 2014		21,440,000
Issuance of common stock for equity awards	$ 593	$ 1	592
Issuance of common stock for equity awards (in shares)	109,441	109,000
Employee Stock Purchase Plan	$ 276		276
Employee Stock Purchase Plan (in shares)		21,000
Stock-based compensation expense	4,583		4,583
Net loss	(46,315)				(46,315)
Other comprehensive income (loss)	(92)			(92)
Balance at Dec. 31, 2015	103,986	$ 22	243,853	(97)	(139,792)
Balance (in shares) at Dec. 31, 2015		21,570,000
Issuance of Common Stock net of issuance costs	147,501	$ 10	147,491
Issuance of Common Stock net of issuance costs (in shares)		10,040,000
Issuance of common stock for equity awards	$ 167		167
Issuance of common stock for equity awards (in shares)	30,195	30,000
Employee Stock Purchase Plan	$ 476		476
Employee Stock Purchase Plan (in shares)		27,000
Stock-based compensation expense	6,770		6,770
Net loss	(71,894)				(71,894)
Other comprehensive income (loss)	26			26
Balance at Dec. 31, 2016	187,032	$ 32	398,757	(71)	(211,686)
Balance (in shares) at Dec. 31, 2016		31,667,000
Issuance of Common Stock net of issuance costs	132,177	$ 6	132,171
Issuance of Common Stock net of issuance costs (in shares)		5,520,000
Issuance of common stock for equity awards	$ 3,858		3,858
Issuance of common stock for equity awards (in shares)	308,011	334,000
Employee Stock Purchase Plan	$ 1,016		1,016
Employee Stock Purchase Plan (in shares)		90,000
Stock-based compensation expense	11,542		11,542
Portion of convertible debt proceeds allocated to equity component	62,466		62,466
Net loss	(137,481)				(137,481)
Other comprehensive income (loss)	(336)			(336)
Balance at Dec. 31, 2017	$ 260,274	$ 38	$ 609,810	$ (407)	$ (349,167)
Balance (in shares) at Dec. 31, 2017		37,611,000