The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

We classify our cash equivalents and available-for-sale investments within Level 1 or Level 2. The fair value of our investment grade corporate debt securities and commercial paper is determined using proprietary valuation models and analytical tools, which utilize market pricing or prices for similar instruments that are both objective and publicly available, such as matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, and offers.

The following table presents the hierarchy for assets measured at fair value on a recurring basis as of March 31, 2019 and December 31, 2018 (in thousands):

	Fair Value Measurements at End of Period Using:
				Quoted Market		Significant		Significant
				Prices for		Other Observable		Unobservable
		Total		Identical Assets		Inputs		Inputs
		Fair Value		(Level 1)		(Level 2)		(Level 3)
As of March 31, 2019
Money market funds		$	40,965	$	40,965	$	—	$	—
U.S. Treasury securities			199,874		199,874		—		—
Commercial paper			26,687		—		26,687		—
Corporate debt securities			83,813		—		83,813		—
As of December 31, 2018
Money market funds		$	17,295	$	17,295	$	—	$	—
U.S. Treasury securities			123,439		123,439		—		—

The Company did not reclassify and investments between levels in the fair value hierarchy during the first quarter of 2019 or 2018.

As of March 31, 2019 and December 31, 2018, the carrying amounts of the Company’s financial instruments, which include cash, interest and securities receivable, accounts payable and accrued expenses, approximate fair values because of their short maturities.

Interest and securities receivable as of March 31, 2019 and December 31, 2018, was $11.2 million and $0.6 million, respectively, and is recorded as a component of prepaid expenses and other current assets on the condensed consolidated balance sheets. Securities receivable reflect the timing differences of maturities or settlements of investments and the ultimate reinvestment of such amounts.

We invest our excess cash in U.S. Treasury securities and debt instruments of corporations and commercial obligations, which we classify as available-for-sale investments. These investments are carried at fair value and are included in the tables above. The Company evaluates securities with unrealized losses to determine whether such losses, if any, are other than temporary. Realized gains and losses are calculated using the specific identification method and recorded as interest income or expense. We do not generally intend to sell the investments and it is not more likely than not that we will be required to sell the investments before recovery of their amortized cost bases, which may be at maturity.

The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in marketable securities and long-term investments for the three months ended March 31, 2019 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury securities	$	199,811	$	69	$	(6	)	$	199,874
Commercial paper		26,694		—		(7	)		26,687
Corporate debt securities		83,738		84		(9	)		83,813
Total marketable securities	$	310,243	$	153	$	(22	)	$	310,374

None of the investments have been in a gross unrealized loss for a period greater than 12 months. At each reporting date, we perform an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of the amortized cost basis. We intend and have the ability to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Further, based on our evaluation, we determined that unrealized losses were not other-than-temporary at March 31, 2019 and December 31, 2018.

Contractual maturities of available-for-sale debt securities, as of March 31, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	261,450
One to two years		48,924
Total	$	310,374

We have the ability, if necessary, to liquidate any of our cash equivalents and short-term investments to meet our liquidity needs in the next 12 months. Accordingly, those investments with contractual maturities greater than one year from the date of purchase are classified as current assets on the accompanying condensed consolidated balance sheets.