2. Investments

As of December 31, 2018, we had primarily invested our excess cash in debt instruments of the U.S. Treasury, financial institutions, corporations, and U.S. government agencies with strong credit ratings and an investment grade rating at or above A-1, P-1 or F-1 by Moody’s, Standard & Poor’s, or S&P, or Fitch, respectively. We have established guidelines relative to diversification and maturities that maintain safety and liquidity. We periodically review and modify these guidelines to maximize trends in yields and interest rates without compromising safety and liquidity.

The following table summarizes the contract maturity of the available-for-sale securities we held as of December 31, 2018:

One year or less	77	%
After one year but within two years	20	%
After two years but within three and one half years	3	%
Total	100	%

As illustrated above, at December 31, 2018, 97 percent of our available-for-sale securities had a maturity of less than two years.

All of our available-for-sale securities are available to us for use in our current operations. As a result, we categorize all of these securities as current assets even though the stated maturity of some individual securities may be one year or more beyond the balance sheet date.

At December 31, 2018, we had an ownership interest of less than 20 percent in four private companies and two public companies with which we conduct business. The privately-held companies are Atlantic Pharmaceuticals Limited, Dynacure SAS, Seventh Sense Biosystems and Suzhou Ribo Life Science Co, Ltd.The publicly traded companies are ATL and ProQR.

The following is a summary of our investments (in thousands):

December 31, 2018	Cost (1)		Gains		Losses			Fair Value
Available-for-sale securities:
Corporate debt securities (2)	$	956,879	$	13	$	(1,858	)	$	955,034
Debt securities issued by U.S. government agencies		168,839		3		(104	)		168,738
Debt securities issued by the U.S. Treasury (2)		244,640		15		(77	)		244,578
Debt securities issued by states of the U.S. and political subdivisions of the states		63,572		—		(323	)		63,249
Total securities with a maturity of one year or less		1,433,930		31		(2,362	)		1,431,599
Corporate debt securities		299,018		194		(1,286	)		297,926
Debt securities issued by U.S. government agencies		107,789		194		(109	)		107,874
Debt securities issued by the U.S. Treasury		15,600		—		(24	)		15,576
Debt securities issued by states of the U.S. and political subdivisions of the states		16,980		—		(287	)		16,693
Total securities with a maturity of more than one year		439,387		388		(1,706	)		438,069
Total available-for-sale securities	$	1,873,317	$	419	$	(4,068	)	$	1,869,668
Equity securities:
Total equity securities included in other current assets (3)	$	1,212		137		—			1,349
Total available-for-sale and equity securities	$	1,874,529	$	556	$	(4,068	)	$	1,871,017

			Gross Unrealized					Estimated
December 31, 2017	Cost (1)		Gains		Losses			Fair Value
Available-for-sale securities:
Corporate debt securities (2)	$	500,599	$	2	$	(752	)	$	499,849
Debt securities issued by U.S. government agencies		83,926		—		(212	)		83,714
Debt securities issued by the U.S. Treasury		29,428		—		(17	)		29,411
Debt securities issued by states of the U.S. and political subdivisions of the states (2)		29,240		4		(122	)		29,122
Total securities with a maturity of one year or less		643,193		6		(1,103	)		642,096
Corporate debt securities		148,663		8		(1,059	)		147,612
Debt securities issued by U.S. government agencies		52,779		—		(168	)		52,611
Debt securities issued by the U.S. Treasury		1,409		—		(2	)		1,407
Debt securities issued by states of the U.S. and political subdivisions of the states		65,550		—		(740	)		64,810
Total securities with a maturity of more than one year		268,401		8		(1,969	)		266,440
Total available-for-sale securities	$	911,594	$	14	$	(3,072	)	$	908,536

(1)	We hold our available-for-sale securities at amortized cost.

(2)	Includes investments classified as cash equivalents on our consolidated balance sheet.

(3)	We recognize our equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer on our consolidated balance sheet.

Investments we consider to be temporarily impaired at December 31, 2018 are as follows (in thousands):

			Less than 12 Months of Temporary Impairment					More than 12 Months of Temporary Impairment					Total Temporary Impairment
	Number of Investments		Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
Corporate debt securities		546	$	1,000,461	$	(1,936	)	$	126,357	$	(1,208	)	$	1,126,818	$	(3,144	)
Debt securities issued by U.S. government agencies		50		161,312		(109	)		34,403		(104	)		195,715		(213	)
Debt securities issued by the U.S. Treasury		36		183,212		(100	)		1,413		(1	)		184,625		(101	)
Debt securities issued by states of the U.S. and political subdivisions of the states		49		13,868		(14	)		62,883		(596	)		76,751		(610	)
Total temporarily impaired securities		681	$	1,358,853	$	(2,159	)	$	225,056	$	(1,909	)	$	1,583,909	$	(4,068	)

We believe that the decline in value of our debt securities is temporary and primarily related to the change in market interest rates since purchase. We believe it is more likely than not that we will be able to hold these securities to maturity. Therefore, we anticipate full recovery of our debt securities’ amortized cost basis at maturity.