4. Fair Value Measurements

Financial assets and liabilities are recorded at fair value. The carrying amounts of certain of our financial instruments, including cash and cash equivalents, restricted cash, short-term investments, receivables from collaborations, prepaid research and development, prepaid expenses and other current assets and accounts payable, accrued research and development, accrued compensation and employee benefits, accrued and other liabilities and deferred revenue and approximate their fair value due to their short maturities. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received in the sale of an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring fair value as follows:

	Level 1 –	Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.
	Level 2 –	Inputs (other than quoted market prices included in Level 1) are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life.
	Level 3 –	Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model.

A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. Where quoted prices are available in an active market, securities are classified as Level 1. We classify money market funds as Level 1. When quoted market prices are not available for the specific security, we estimate fair value by using quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs obtained from various third-party data providers, including but not limited to, benchmark yields, interest rate curves, reported trades, broker/dealer quotes and market reference data. We classify our corporate notes, commercial paper, U.S. Treasury bills and government agency securities and foreign currency forward contracts as Level 2. Level 2 inputs for the valuations are limited to quoted prices for similar assets or liabilities in active markets and inputs other than quoted prices that are observable for the asset or liability. Mid-market pricing is used as a practical expedient for fair value measurements. The fair value measurement of any asset or liability must reflect the non-performance risk of the entity and the counterparty to the transaction. Therefore, the impact of the counterparty’s creditworthiness, when in an asset position, and our creditworthiness, when in a liability position, has also been factored into the fair value measurement.

In certain cases where there is limited activity or less transparency around inputs to valuation, the related assets or liabilities are classified as Level 3. Our embedded derivative liabilities are measured at fair value using a Monte Carlo simulation model and are included as a component of other long-term liabilities on the consolidated balance sheets. The embedded derivative liabilities are subject to remeasurement at the end of each reporting period, with changes in fair value recognized as a component of interest and other income (expense), net, in our condensed consolidated statements of operations. The assumptions used in the Monte Carlo simulation model include: 1) our estimates of both the probability and timing of regulatory approval of Andexxa by geographical region and other related events; 2) the probability-weighted net sales of Andexxa; 3) our risk adjusted discount rate that includes a company specific risk premium; 4) our cost of debt; 5) volatility; 6) the probability of a change in control occurring during the term of the note; and 7) the probability of an event of default. The valuation of our embedded derivative liabilities is most sensitive to the probability of Andexxa achieving regulatory approval given the binary nature of such an approval event and the correlation to other assumptions included in the model.

There were no transfers between Level 1, Level 2 and Level 3 during the periods presented.

In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3. Our noncontrolling interest in SRX Cardio includes the fair value of the contingent milestone and royalty payments, which is valued based on Level 3 inputs. See Note 7, "Asset Acquisition and License Agreements," to these condensed consolidated financial statements for further information.

The following table sets forth the fair value of our financial assets and liabilities (excluding consolidated VIE’s cash), allocated into Level 1, Level 2 and Level 3, that was measured on a recurring basis (in thousands):

	March 31, 2018
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	19,507	$	—	$	—	$	19,507
Corporate notes and commercial paper	—			260,950		—		260,950
U.S. Treasury bills and government agency securities		—		154,412		—		154,412
Total financial assets	$	19,507	$	415,362	$	—	$	434,869
Financial Liabilities:
Embedded derivatives liabilities	$	—	$	—	$	7,220	$	7,220

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	31,836	$	—	$	—	$	31,836
Corporate notes and commercial paper		—		313,164		—		313,164
U.S. Treasury bills and government agency securities		—		170,458		—		170,458
Total financial assets	$	31,836	$	483,622	$	—	$	515,458
Financial Liabilities:
Embedded derivatives liabilities	$	—	$	—	$	8,854	$	8,854

Level 3 liabilities are comprised of embedded derivatives liabilities as described in Note 8 “Asset Acquisition and License Agreements.” The following table sets forth a summary of the changes in the estimated fair value of our embedded derivatives liabilities, which were measured at fair value on a recurring basis (in thousands):

Balance as of December 31, 2017	$	8,854
Net decrease in fair value included in interest and other income, net		(1,634	)
Balance as of March 31, 2018	$	7,220

We estimate the fair values of our corporate notes and commercial paper and U.S government agency securities by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry-standard valuation models, including both income- and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads, benchmark securities, prepayment/default projections based on historical data and other observable inputs.

There were no transfers between any of the levels of the fair value hierarchy during the periods presented.