Note 3 - Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Notes to Financial Statements

Fair Value Disclosures [Text Block]

3. Fair Value Measurements

Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants

on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The fair value hierarchy based on

three

levels of inputs, of which the

first

two

are considered observable and the last unobservable, that

may

be used to measure fair value is as follows:

• Level 1 — Observable inputs such as quoted prices in active markets for identical assets or liabilities.

• Level 2 — Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

• Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

We

measure the following financial assets at fair value on a recurring basis. The fair values of these financial assets were as follows (in thousands):

			Fair Value Measurements at Reporting Date Using
	Balance at December 31 , 2017		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Money market funds	$	91,386	$	91,386	$	—	$	—
U.S. corporate debt securities		17,520		—		17,520		—
U.S. commercial paper		39,863		—		39,863		—
Foreign commercial paper		19,854		—		19,854		—
Total	$	168,623	$	91,386	$	77,237	$	—

			Fair Value Measurements at Reporting Date Using
	Balance at December 31 , 2016		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Money market funds	$	11,493	$	11,493	$	—	$	—
U.S. corporate debt securities		12,039		—		12,039		—
Foreign corporate debt securities		14,057		—		14,057		—
U.S. commercial paper		8,635		—		8,635		—
Foreign commercial paper		2,993		—		2,993		—
Total	$	49,217	$	11,493	$	37,724	$	—

There were

no

significant transfers between

Level

1

and

2

investments during the years ended

December 31, 2017

and

December 31, 2016.

As of

December 31, 2017,

cash equivalents included

$49.4

million of available-for-sale securities with contractual maturities of

three

months or less, and short-term investments included

$27.8

million of available-for-sale securities with contractual maturities of

three

months to

one

year. As of

December 31, 2016,

we had short-term investments consisting of

$37.7

million of available-for-sale securities with contractual maturities of

three

months to

one

year. The money market funds as of

December 31, 2017

and

2016

are included in cash and cash equivalents on the consolidated balance sheets.

Unrealized gains and losses

associated with our investments are reported in accumulated other comprehensive loss. For the years ended

December 31, 2017

and

2016,

we recorded

$7,000

and

$23,000,

respectively, in net unrealized gains associated with our short-term investments. For the year ended

December 31, 2015,

we recorded

$40,000

in net unrealized losses associated with our short-term investments.

Realized gains and losses associated with our investments, if any, are reported in

the statements of operations and comprehensive loss. There were

no

realized gains or losses for the years ended

December 31, 2017,

2016

and

2015.