The carrying value and amortized cost of the Company’s investments, summarized by major security type, consisted of the following (in thousands):

	March 31, 2020
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
U.S. Treasury notes	$	238,541	$	1,588	$	—		$	240,129
Government sponsored enterprise securities		71,030		292		—			71,322
Corporate debt securities		112,075		74		(491	)		111,658
Commercial paper		55,204		40		(6	)		55,238
	$	476,850	$	1,994	$	(497	)	$	478,347

	December 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
U.S. Treasury notes	$	228,609	$	7	$	(13	)	$	228,603
Government sponsored enterprise securities		88,920		5		(30	)		88,895
Corporate debt securities		87,785		262		(8	)		88,039
Commercial paper		102,151		61		—			102,212
	$	507,465	$	335	$	(51	)	$	507,749

The Company has classified all of its available-for-sale investment securities, including those with maturities beyond one year, as current assets on its consolidated balance sheets based on the highly liquid nature of the investment securities and because these investment securities are considered available for use in current operations. As of March 31, 2020 and December 31, 2019, the Company held $3.0 million and $24.3 million, respectively, of available-for-sale investment securities with contractual maturity dates of more than one year and less than two years. The Company has classified all equity securities as other assets on its Consolidated Balance Sheets.

At March 31, 2020 and December 31, 2019, the Company had 21 and 20 available-for-sale investment securities, respectively, in an unrealized loss position. The following table presents gross unrealized losses and fair value for those available-for-sale investment securities that were in an unrealized loss position as of March 31, 2020 and December 31, 2019, aggregated by investment category and length of time that the individual securities have been in a continuous loss position (in thousands):

	Less Than 12 Months					12 Months or Greater				Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
March 31, 2020:
Corporate debt securities	$	89,774	$	(491	)	$	—	$	—	$	89,774	$	(491	)
Commercial paper		24,806		(6	)		—		—		24,806		(6	)
Total	$	114,580	$	(497	)	$	—	$	—	$	114,580	$	(497	)

	Less Than 12 Months					12 Months or Greater				Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
December 31, 2019:
U.S. Treasury notes	$	148,397	$	(13	)	$	—	$	—	$	148,397	$	(13	)
Government sponsored enterprise securities		61,103		(30	)		—		—		61,103		(30	)
Corporate debt securities		11,334		(8	)		—		—		11,334		(8	)
Total	$	220,834	$	(51	)	$	—	$	—	$	220,834	$	(51	)

As described further in Note 10, Recent Accounting Pronouncements the Company adopted FASB Accounting Standards Codification 326-30, Financial Instruments – Credit Losses (“ASC 326-30”) as of January 1, 2020. ASC 326-30 requires the Company to determine whether a decline in the fair value below the amortized cost basis of available-for-sale securities is due to credit-related factors. At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are the result of credit losses. Impairment is assessed at the individual security level. Factors considered in determining whether a loss resulted from a credit loss or other factors include the Company’s intent and ability to hold the investment until the recovery of its amortized cost basis, the extent to which the fair value is less than the amortized cost basis, the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, any historical failure of the issuer to make scheduled interest or principal payments, any changes to the rating of the security by a rating agency, any adverse legal or regulatory events affecting the issuer or issuer’s industry, any significant deterioration in economic conditions.

The Company does not intend to sell the investments in unrealized loss position and it is unlikely that the Company will be required to sell the investments before the recovery of their amortized cost basis. Based on its evaluation, the Company determined its year-to-date credit losses related to its available-for-sale securities were immaterial at March 31, 2020.

Although the Company has not historically experienced significant losses on its investments, the Company’s exposure may increase due to uncertainties associated with the global economic recession and other disruptions resulting from the COVID-19 pandemic.