Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value of Financial Liabilities

The following table sets forth the fair value of the Company’s financial liabilities by level within the fair value hierarchy (in thousands):

	At December 31, 2017
	Level 1		Level 2		Level 3		Total
2017 Placement Warrants Liability	$	—	$	—	$	12,487	$	12,487
2016 Placement Warrants Liability		—		—		9,933		9,933
Total Warrant Liability	$	—	$	—	$	22,420	$	22,420

	At December 31, 2016
	Level 1		Level 2		Level 3		Total
2016 Placement Warrants Liability	$	—	$	—	$	2,723	$	2,723

Summary of Changes in Fair Value of Level 3 Financial Liabilities

The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial liabilities (in thousands):

	Year Ended December 31,
	2017			2016			2015
Fair value, beginning of period	$	2,723		$	—		$	138
Issuance of 2017 Placement Warrants		3,373			—			—
Issuance of 2016 Placement Warrants		—			2,726			—
Change in fair value of Level 3 financial    liabilities		16,598			(3	)		(45	)
Conversion of convertible preferred stock    warrants to common stock warrants		—			—			(93	)
Fair value of 2017 Placement Warrants    at exercise		(74	)		—			—
Fair value of 2016 Placement Warrants    at exercise		(200	)		—			—
Fair value, end of period	$	22,420		$	2,723		$	—