Note 2 - Investments and Fair Value Measurement (Tables)

3 Months Ended

Mar. 31, 2018

Cash, Cash Equivalents and Investments [Table Text Block]

	As of March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Cash and cash equivalents:
Cash	$	20,392	$	—	$	—	$	20,392
U.S. government agency securities		23,211		—		—		23,211
Total cash and cash equivalents	$	43,603	$	—	$	—	$	43,603

Marketable securities:
U.S. government agency securities	$	7,573	$	—	$	—	$	7,573
Total marketable securities	$	7,573	$	—	$	—	$	7,573

Total cash, cash equivalents and investments	$	51,176	$	—	$	—	$	51,176

	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Cash and cash equivalents:
Cash	$	29,765	$	—	$	—	$	29,765
U.S. government agency securities		23,137		—		—		23,137
Total cash and cash equivalents	$	52,902	$	—	$	—	$	52,902

Marketable securities:
U.S. government agency securities	$	7,567	$	—	$	—	$	7,567
Total marketable securities	$	7,567	$	—	$	—	$	7,567

Total cash, cash equivalents and investments	$	60,469	$	—	$	—	$	60,469

Fair Value Measurements, Recurring and Nonrecurring [Table Text Block]

	As of March 31, 2018
	Fair Value		Level I		Level II		Level III
Assets
U.S. government agency obligations	$	30,784	$	—	$	30,784	$	—
Total assets measured at fair value	$	30,784	$	—	$	30,784	$	—

Liabilities
Contingent put option liability	$	186	$	—	$	—	$	186
Total liabilities measured at fair value	$	186	$	—	$	—	$	186

	As of December 31, 2017
	Fair Value		Level I		Level II		Level III
Assets
U.S. government agency obligations	$	30,704	$	—	$	30,704	$	—
Total assets measured at fair value	$	30,704	$	—	$	30,704	$	—

Liabilities
Contingent put option liability	$	207	$	—		—	$	207
Total liabilities measured at fair value	$	207	$	—	$	—	$	207

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]

	Three Months Ended March 31, 2018
Fair value—beginning of period	$	207
Change in fair value of contingent put option associated with Amended Loan Agreement		(21	)
Fair value—end of period	$	186

	Three Months Ended March 31, 2017
Fair value—beginning of period	$	412
Change in fair value of PIPE warrants		7
Change in fair value of contingent put option associated with Amended Loan Agreement		190
Fair value—end of period	$	609

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

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