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 September 30, 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 September 30, 2019
Money market funds		$	35,756	$	35,756	$	—	$	—
U.S. Treasury and agency securities			199,500		199,500		—		—
Commercial paper			30,985		—		30,985		—
Corporate debt securities			99,892		—		99,892		—
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 any investments between levels in the fair value hierarchy during the periods presented.

As of September 30, 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 September 30, 2019 and December 31, 2018, was $1.2 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 believe that our term loan facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of the term loan facility approximates fair value. We estimate the fair value of long-term debt utilizing an income approach. We use a present value calculation to discount principal and interest payments and the final maturity payment on these liabilities using a discounted cash flow model based on observable inputs. The debt instrument is then discounted based on what the current market rates would be as of the reporting date. Based on the assumptions used to value these liabilities at fair value, the debt instrument is categorized as Level 2 in the fair value hierarchy.

We invest our excess cash in U.S. Treasury and agency securities and debt instruments of corporations and commercial obligations, which are classified 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 as of September 30, 2019 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury and agency securities	$	185,844	$	145	$	(4	)	$	185,985
Commercial paper		29,887		—		—			29,887
Corporate debt securities		99,578		319		(5	)		99,892
Total marketable securities	$	315,309	$	464	$	(9	)	$	315,764

As of September 30, 2019, the Company classified $14.6 million of assets with original maturities of 90 days or less as cash equivalents. 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 as of September 30, 2019.

Contractual maturities of available-for-sale debt securities, as of September 30, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	290,813
One to two years		24,951
Total	$	315,764

We have the ability, if necessary, to liquidate any of our cash equivalents and marketable securities 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.