2. Investments

As of December 31, 2017, 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, 2017:

One year or less	71	%
After one year but within two years	23	%
After two years but within three and one half years	6	%
Total	100	%

As illustrated above, at December 31, 2017, 94 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, 2017, we had an ownership interest of less than 20 percentin five private companies and one public company with which we conduct business. The privately-held companies are Atlantic Pharmaceuticals Limited, Dynacure SAS, Kastle Therapeutics, Seventh Sense Biosystems and Suzhou Ribo Life Science CO. The publicly traded company is Antisense Therapeutics Limited, or ATL. We account for our equity investments in the privately-held companies under the cost method of accounting and we account for our equity investment in the publicly traded company at fair value. We record unrealized gains and losses as a separate component of comprehensive income (loss) and include net realized gains and losses in gain (loss) on investments.

During 2015 and 2017, we realized a net gain on our investment in Regulus of $20.2 million and $0.4 million, respectively, when we sold our stock in Regulus.

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

			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

			Gross Unrealized					Estimated
December 31, 2016	Cost (1)		Gains		Losses			Fair Value
Available-for-sale securities:
Corporate debt securities	$	195,087	$	25	$	(161	)	$	194,951
Debt securities issued by U.S. government agencies		26,548		—		(10	)		26,538
Debt securities issued by the U.S. Treasury		29,298		2		(14	)		29,286
Debt securities issued by states of the U.S. and political subdivisions of the states (2)		72,775		2		(134	)		72,643
Total securities with a maturity of one year or less		323,708		29		(319	)		323,418
Corporate debt securities		202,408		36		(1,174	)		201,270
Debt securities issued by U.S. government agencies		28,807		1		(167	)		28,641
Debt securities issued by states of the U.S. and political subdivisions of the states		36,816		1		(349	)		36,468
Total securities with a maturity of more than one year		268,031		38		(1,690	)		266,379
Total available-for-sale securities	$	591,739	$	67	$	(2,009	)	$	589,797
Equity securities:
Regulus Therapeutics Inc.	$	2,133	$	281	$	—		$	2,414
Total equity securities	$	2,133	$	281	$	—		$	2,414
Total available-for-sale and equity securities	$	593,872	$	348	$	(2,009	)	$	592,211

(1)

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

(2)

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

Investments we consider to be temporarily impaired at December 31, 2017 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		476	$	551,446	$	(1,236	)	$	74,987	$	(575	)	$	626,433	$	(1,811	)
Debt securities issued by U.S. government agencies		45		107,788		(262	)		27,538		(118	)		135,326		(380	)
Debt securities issued by the U.S. Treasury		7		30,818		(19	)		—		—			30,818		(19	)
Debt securities issued by states of the U.S. and political subdivisions of the states		60		62,519		(545	)		24,572		(317	)		87,091		(862	)
Total temporarily impaired securities		588	$	752,571	$	(2,062	)	$	127,097	$	(1,010	)	$	879,668	$	(3,072	)

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.