Fair Value Measurements

6 Months Ended

12 Months Ended

Jun. 30, 2018

Dec. 31, 2017

Fair value

The fair value of our financial assets reflects our estimate of amounts that we would have received in connection with the sale of such asset in an orderly transaction between market participants at the measurement date. In connection with measuring the fair value of our assets, we seek to maximize the use of observable inputs (market data obtained from independent sources) and to minimize the use of unobservable inputs (our assumptions about how market participants would price assets and liabilities). We use the following fair value hierarchy to classify assets based on the observable inputs and unobservable inputs we used to value our assets and liabilities:


•	Level 1—quoted prices (unadjusted) in active markets for identical assets.


•	Level 2—quoted prices for similar assets in active markets or inputs that are observable for the asset, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument.


•	Level 3—unobservable inputs based on our assumptions used to measure assets at fair value.

The following tables summarize our assets that are measured at fair value as of June 30, 2018 and December 31, 2017 (in thousands):



Description	Balance as of June 30, 2018		Level 1		Level 2		Level 3
Assets:
Cash and money market funds	$	24,601	$	24,601	$	—	$	—
Corporate debt securities (including commercial paper)	21,046		—		21,046		—
U.S. government securities	7,981		—		7,981		—
Total	$	53,628	$	24,601	$	29,027	$	—



Description	Balance as of December 31, 2017		Level 1		Level 2		Level 3
Assets:
Cash and money market funds	$	15,703	$	15,703	$	—	$	—
Corporate debt securities (including commercial paper)	35,531		—		35,531		—
U.S. government securities	15,969		—		15,969		—
Total	$	67,203	$	15,703	$	51,500	$	—

Fair value Measurements

The fair value of our financial assets reflects our estimate of amounts that we would have received in connection with the sale of such assets in an orderly transaction between market participants at the measurement date. In connection with measuring the fair value of our assets, we seek to maximize the use of observable inputs (market data obtained from independent sources) and to minimize the use of unobservable inputs (our assumptions about how market participants would price assets and liabilities). We use the following fair value hierarchy to classify assets based on the observable inputs and unobservable inputs we used to value our assets and liabilities:


•	Level 1 — quoted prices (unadjusted) in active markets for identical assets.


•	Level 2 — quoted prices for similar assets in active markets or inputs that are observable for the asset, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument.


•	Level 3 — unobservable inputs based on our assumptions used to measure assets at fair value.

The following tables summarize our assets that are measured at fair value as of December 31, 2017 and 2016 (in thousands):



Description	Balance as of December 31, 2017		Level 1		Level 2		Level 3
Assets:
Cash and money market funds	$	15,703	$	15,703	$	—	$	—
Corporate debt securities (including commercial paper)	35,531		—		35,531		—
U.S. government securities	15,969		—		15,969		—
Total assets	$	67,203	$	15,703	$	51,500	$	—



Description	Balance as of December 31, 2016		Level 1		Level 2		Level 3
Assets:
Cash and money market funds	$	43,930	$	43,930	$	—	$	—
Corporate debt securities (including commercial paper)	48,466		—		48,466		—
U.S. government securities	21,992		—		21,992		—
Total assets	$	114,388	$	43,930	$	70,458	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

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