3. Investments

As of September 30, 2018, we had invested our excess cash primarily 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 September 30, 2018:

One year or less	80%
After one year but within two years	15%
After two years but within three years	5%
Total	100%

As illustrated above, at September 30, 2018, 95 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 September 30, 2018, we had an ownership interest of less than 20 percent in four private companies and one public company 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 company is Antisense Therapeutics Limited.

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

			Gross Unrealized
September 30, 2018	Cost (1)		Gains		Losses			Estimated Fair Value
Available-for-sale securities:
Corporate debt securities (2)	$	841,397	$	23	$	(1,264	)	$	840,156
Debt securities issued by U.S. government agencies		143,008		2		(169	)		142,841
Debt securities issued by the U.S. Treasury (2)		154,280		—		(89	)		154,191
Debt securities issued by states of the U.S. and political subdivisions of the states (2)		49,873		—		(330	)		49,543
Total securities with a maturity of one year or less		1,188,558		25		(1,852	)		1,186,731
Corporate debt securities		240,208		16		(1,420	)		238,804
Debt securities issued by U.S. government agencies		32,435		—		(109	)		32,326
Debt securities issued by the U.S. Treasury		9,872		—		(10	)		9,862
Debt securities issued by states of the U.S. and political subdivisions of the states		31,666		—		(551	)		31,115
Total securities with a maturity of more than one year		314,181		16		(2,090	)		312,107
Total available-for-sale securities	$	1,502,739	$	41	$	(3,942	)	$	1,498,838

			Gross Unrealized
December 31, 2017	Cost (1)		Gains		Losses			Estimated Fair Value
Available-for-sale securities:
Corporate debt securities	$	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)

Our available-for-sale securities are held at amortized cost.

(2)

Includes investments classified as cash equivalents on our condensed consolidated balance sheet.

The following is a summary of our investments we consider to be temporarily impaired at September 30, 2018. We believe that the decline in value of these securities is temporary and is 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 our debt securities to maturity. Therefore, we anticipate full recovery of our debt securities’ amortized cost basis at maturity.

			Less than 12 Months of Temporary Impairment					More than 12 Months of Temporary Impairment					Total Temporary Impairment
(In thousands)	Number of Investments		Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
Corporate debt securities		495	$	889,172	$	(1,687	)	$	83,706	$	(997	)	$	972,878	$	(2,684	)
Debt securities issued by U.S. government agencies		51		132,231		(139	)		26,862		(139	)		159,093		(278	)
Debt securities issued by the U.S. Treasury		32		164,053		(99	)		—		—			164,053		(99	)
Debt securities issued by states of the U.S. and political subdivisions of the states		48		25,687		(191	)		51,747		(690	)		77,434		(881	)
Total temporarily impaired securities		626	$	1,211,143	$	(2,116	)	$	162,315	$	(1,826	)	$	1,373,458	$	(3,942	)