Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of valuation of the Company's investments and financial instruments that are measured at fair value on a recurring basis

The following table summarizes the valuation of the Company’s financial instruments that are measured at fair value on a recurring basis:

	September 30, 2018								December 31, 2017
			Fair value measurement category								Fair value measurement category
(In thousands)	Total		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
Assets:
Money market funds	$	18,684	$	18,684	$	—	$	—	$	—	—		—		—
Repurchase agreements	10,002		—		10,002		—		—		—		—		—
Commercial paper	21,832		—		21,832		—		—		—		—		—
Corporate notes	13,047		—		13,047		—		—		—		—		—
U.S. government securities	3,173		—		3,173		—		—		—		—		—
U.S. asset-backed securities	8,424		—		8,424		—		—		—		—		—
	$	75,162	$	18,684	$	56,478	$	—	$	—	$	—	$	—	$	—
Liabilities:
Warrant liabilities	$	—	$	—	$	—	$	—	$	1,014	$	—	$	1,014	$	—

Summary of change in estimated fair value of warrant liabilities

The following table summarizes the change in the estimated fair value of the Company’s outstanding warrant liabilities as of September 30, 2018: 

Warrant Liabilities (In thousands)
Balance at December 31, 2017	$	1,014
Increase in fair value (net of expired warrants)	2,524
Warrant exercise	(3,538		)
Balance at September 30, 2018	$	—