Note 2 - Investments and Fair Value Measurement (Tables)

12 Months Ended

Dec. 31, 2017

Cash, Cash Equivalents and Investments [Table Text Block]

	As of December 31 , 201 7
	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

	As of December 31 , 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Cash and cash equivalents:
Cash	$	49,833	$	—	$	—	$	49,833
U.S. government agency securities		30,474		3		—		30,477
Total cash and cash equivalents		80,307		3		—		80,310
Total cash, cash equivalents and investments	$	80,307	$	3	$	—	$	80,310

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

	As of December 31 , 201 7
	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

	As of December 31 , 2016
	Fair Value		Level I		Level II		Level III
Assets
U.S. government agency obligations	$	30,477	$	—	$	30,477	$	—
Total assets measured at fair value	$	30,477	$	—	$	30,477	$	—

Liabilities
PIPE warrants	$	288		—		—	$	288
Contingent put option liability		124		—		—		124
Total liabilities measured at fair value	$	412	$	—	$	—	$	412

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

	Year Ended December 31, 201 7
Fair value —beginning of period	$	412
Expiration of fair value of PIPE warrants		(288	)
Change in fair value of contingent put option associated with Original Loan Agreement		83
Fair value —end of period	$	207

	Year Ended December 31, 201 6
Fair value —beginning of period	$	1,179
Change in fair value of PIPE warrants		(625	)
Change in fair value of contingent put option associated with Original Loan Agreement		(142	)
Fair value —end of period	$	412

- 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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