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.

The Company classifies its cash equivalents and available-for-sale investments within Level 1 or Level 2. The fair value of the Company’s 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 June 30, 2020 and December 31, 2019 (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 June 30, 2020
Money market funds		$	365,982	$	365,982	$	—	$	—
U.S. Treasury and agency securities			6,175		6,175		—		—
Commercial paper			25,213		—		25,213		—
Corporate debt securities			82,914		—		82,914		—
As of December 31, 2019
Money market funds		$	82,125	$	82,125	$	—	$	—
U.S. Treasury and agency securities			91,717		91,717		—		—
Commercial paper			37,411		—		37,411		—
Corporate debt securities			156,277		—		156,277		—

The Company did not reclassify any investments between levels in the fair value hierarchy during the periods presented.

As of June 30, 2020 and December 31, 2019, the carrying amounts of the Company’s financial instruments, which include cash, interest receivable, accounts payable and accrued expenses, approximate fair values because of their short maturities.

Interest receivable as of June 30, 2020 and December 31, 2019, was $0.6 million and $1.5 million, respectively, and is recorded as a component of prepaid expenses and other current assets on the condensed consolidated balance sheets.

The Company believes that its Credit Facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of the Credit Facility approximates fair value. The Company estimates the fair value of long-term debt utilizing an income approach. The Company uses 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.

As of June 30, 2020 the fair value of the Company’s 2027 Notes was $200.5 million. The fair value was determined on the basis of market prices observable for similar instruments and is considered Level 2 in the fair value hierarchy (see Note 5).

The Company invests its 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 due to credit-related factors. Realized gains and losses are calculated using the specific identification method and recorded as interest income or expense. The Company does not generally intend to sell the investments and it is not more likely than not that the Company 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 June 30, 2020 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized		Total
	Cost		Gains		Losses		Fair Value
Marketable securities
U.S. Treasury and agency securities	$	6,160	$	15	$	—	$	6,175
Commercial paper		25,213		—		—		25,213
Corporate debt securities		82,335		579		—		82,914
Total marketable securities	$	113,708	$	594	$	—	$	114,302

At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are due to credit-related factors. The Company records an allowance for credit losses when unrealized losses are due to credit-related factors. Factors considered when evaluating available-for-sale investments for impairment include the severity of the impairment, changes in underlying credit ratings, the financial condition of the issuer, the probability that the scheduled cash payments will continue to be made and the Company’s intent and ability to hold the investment until recovery of the amortized cost basis. The Company intends and has the ability to hold its investments in unrealized loss positions until their amortized cost basis has been recovered. As of June 30, 2020, there were no material declines in the market value of the Company’s available-for-sale investments due to credit-related factors.

Contractual maturities of available-for-sale debt securities, as of June 30, 2020, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	100,376
One to two years		13,926
Total	$	114,302

The Company has the ability, if necessary, to liquidate any of its cash equivalents and marketable securities to meet its 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.