Financial Instruments

Cash and Cash Equivalents and Investments

The following tables summarize our cash and available-for-sale securities by significant investment category at December 31, 2019 and 2018 (in thousands):



	December 31, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Cash and Cash Equivalents		Current Marketable Securities		Non- Current Marketable Securities
Cash and cash equivalents	$	225,874	$	—	$	—		$	225,874	$	225,874	$	—	$	—
Available-for-sale:
Level 2:
Certificates of deposit	82,028		79		(6		)	82,101		10,002		69,197		2,902
U.S. treasury securities	117,891		260		(2		)	118,149		—		110,186		7,963
Debt securities of U.S. government agencies and corporate entities	834,187		2,708		(32		)	836,863		—		687,741		149,122
	$	1,259,980	$	3,047	$	(40	)	$	1,262,987	$	235,876	$	867,124	$	159,987

	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value		Cash and Cash Equivalents		Current Marketable Securities		Non- Current Marketable Securities
Cash and cash equivalents	$	658,365	$	20	$	(21	)	$	658,364	$	658,364	$	—	$	—
Available-for-sale:
Level 2:
Certificates of deposit	173,102		42		(36		)	173,108		—		157,920		15,188
U.S. treasury securities	152,205		18		(48		)	152,175		—		152,175		—
Debt securities of U.S. government agencies and corporate entities	712,065		40		(1,335		)	710,770		—		552,968		157,802
	$	1,695,737	$	120	$	(1,440	)	$	1,694,417	$	658,364	$	863,063	$	172,990

The amortized cost and estimated fair value of marketable securities, by contractual maturity at December 31, 2019 and 2018 are as follows (in thousands):



	December 31, 2019
	Amortized Cost		Estimated Fair Value
Due in one year or less	$	864,958	$	867,124
Due after one year through five years	159,148		159,987
Total	$	1,024,106	$	1,027,111
	December 31, 2018
	Amortized Cost		Estimated Fair Value
Due in one year or less	$	864,003	$	863,063
Due after one year through five years	173,369		172,990
Total	$	1,037,372	$	1,036,053

At December 31, 2019, there were no available-for-sale securities in our total investment portfolio that were in a continuous unrealized loss position for more than 12 months. At December 31, 2018, we held 25 available-for-sale securities, or an estimated fair value of $82.8 million, out of our total investment portfolio that were in a continuous unrealized loss position for more than 12 months with a gross unrealized loss of $0.4 million. We concluded that the net declines in market value of our available-for-sale securities investment portfolio were temporary in nature and did not consider any of our investments to be other-than-temporarily impaired. In accordance with our investment policy, we place investments in investment grade securities with high credit quality issuers, and generally limit the amount of credit exposure to any one issuer. We evaluate securities for other-than-temporary impairment at the end of each reporting period. Impairment is evaluated considering numerous factors, and their relative significance varies depending on the situation. Factors considered include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the issuer, and our intent and ability to hold the investment to allow for an anticipated recovery in fair value. Furthermore, the aggregate of individual unrealized losses that had been outstanding for 12 months or less was not significant as of December 31, 2019, 2018. and 2017. We neither intend to sell these investments nor conclude that we are more-likely-than-not that we will have to sell them before recovery of their carrying values. We also believe that we will be able to collect both principal and interest amounts due to us at maturity.