Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 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

	December 31, 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	$	5,838	$	5,838	$	—	$	—	$	—	$	—	$	—	$	—
Repurchase agreements	5,000		—		5,000		—		—		—		—		—
Commercial paper	30,710		—		30,710		—		—		—		—		—
Corporate notes	13,144		—		13,144		—		—		—		—		—
U.S. government securities	10,166		—		10,166		—		—		—		—		—
U.S. asset-backed securities	10,618		—		10,618		—		—		—		—		—
	$	75,476	$	5,838	$	69,638	$	—	$	—	$	—	$	—	$	—
Liabilities:
Warrant liabilities	$	—	$	—	$	—	$	—	$	1,014	$	—	$	1,014	$	—

Summary of change in estimated fair values of warrant liabilities

The following table summarizes the change in the estimated fair value of the Company’s warrant liabilities: 

Warrant Liabilities (In thousands)
Balance at December 31, 2016	$	757
Increase in fair value	257
Balance at December 31, 2017	1,014
Increase in fair value (net of expired warrants)	2,524
Warrant exercise	(3,538		)
Balance at December 31, 2018	$	—