Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Summary of financial assets and liabilities measured on recurring basis

	December 31, 2017								December 31, 2016
	Level I		Level II		Level III		Total			Level I		Level II		Level III		Total
	(in thousands)
Financial Assets:
Money market deposits	$	3,477	$	—	$	—	$	3,477		$	10,777	$	—	$	—	$	10,777
U.S. Treasury securities		67,026		—		—		67,026			57,442		—		—		57,442
U.S. agency securities		—		27,072		—		27,072			—		56,114		—		56,114
Municipal securities		—		12,149		—		12,149			—		17,304		—		17,304
Total financial assets	$	70,503	$	39,221	$	—	$	109,724		$	68,219	$	73,418	$	—	$	141,637
Current Liabilities:
Warrants	$		$	—	$	2,644	$	2,644		$	—	$	—	$	3,792	$	3,792
Total financial liabilities	$	—	$	—	$	2,644	$	2,644		$	—	$	—	$	3,792	$	3,792

Summary of quantitative information about inputs and valuation methodologies

					Weighted Average
					Interest on
	Fair Value at			Significant	Discount Rate
	December 31, 2017		Valuation Methodology	Unobservable Input	(range, if applicable)
	(in thousands)				(in thousands)
Warrants	$	2,644	Black-Scholes Option Pricing Model	Volatility		40.93	%

Warrants

Rollforward of fair value determined by Level 3 inputs

	Warrants
	2017		2016
	(in thousands)
Beginning balance	$	3,792	$	3,649
Change in fair value		(1,148)		143
Ending balance	$	2,644	$	3,792