Investments

As of March 31, 2016, 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 March 31, 2016:

One year or less	51%
After one year but within two years	32%
After two years but within three and a half years	17%
Total	100%

As illustrated above, at March 31, 2016, 83 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 March 31, 2016, we had an ownership interest of less than 20 percent in one private company and two public companies with which we conduct business. The privately-held company is Atlantic Pharmaceuticals Limited and the publicly-traded companies are Antisense Therapeutics Limited and Regulus. We account for equity investments in the privately-held company under the cost method of accounting and we account for equity investments in the publicly-traded companies 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.

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

			Gross Unrealized
March 31, 2016	Cost		Gains		Losses			Estimated Fair Value
Available-for-sale securities (1):
Corporate debt securities	$	161,995	$	29	$	(59	)	$	161,965
Debt securities issued by U.S. government agencies		42,545		3		(1	)		42,547
Debt securities issued by the U.S. Treasury		13,101		3		(3	)		13,101
Debt securities issued by states of the United States and political subdivisions of the states (2)		78,231		14		(97	)		78,148
Total securities with a maturity of one year or less		295,872		49		(160	)		295,761
Corporate debt securities		242,995		608		(416	)		243,187
Debt securities issued by U.S. government agencies		36,951		2		(12	)		36,941
Debt securities issued by states of the United States and political subdivisions of the states		40,201		26		(82	)		40,145
Total securities with a maturity of more than one year		320,147		636		(510	)		320,273
Total available-for-sale securities	$	616,019	$	685	$	(670	)	$	616,034
Equity securities:
Regulus Therapeutics Inc.	$	7,162	$	12,541	$	—		$	19,703
Total equity securities	$	7,162	$	12,541	$	—		$	19,703
Total available-for-sale and equity securities	$	623,181	$	13,226	$	(670	)	$	635,737

			Gross Unrealized
December 31, 2015	Cost		Gains		Losses			Estimated Fair Value
Available-for-sale securities (1):
Corporate debt securities	$	181,670	$	5	$	(250	)	$	181,425
Debt securities issued by U.S. government agencies		50,559		1		(19	)		50,541
Debt securities issued by the U.S. Treasury		2,604		—		(3	)		2,601
Debt securities issued by states of the United States and political subdivisions of the states (2)		79,414		18		(88	)		79,344
Total securities with a maturity of one year or less		314,247		24		(360	)		313,911
Corporate debt securities		258,703		3		(1,705	)		257,001
Debt securities issued by U.S. government agencies		38,956		—		(244	)		38,712
Debt securities issued by states of the United States and political subdivisions of the states		48,552		3		(243	)		48,312
Total securities with a maturity of more than one year		346,211		6		(2,192	)		344,025
Total available-for-sale securities	$	660,458	$	30	$	(2,552	)	$	657,936
Equity securities:
Regulus Therapeutics Inc.	$	7,162	$	17,630	$	—		$	24,792
Total equity securities	$	7,162	$	17,630	$	—		$	24,792
Total available-for-sale and equity securities	$	667,620	$	17,660	$	(2,552	)	$	682,728

(1)

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

(2)

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

Investments we consider to be temporarily impaired at March 31, 2016 were 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		168	$	203,743	$	(442	)	$	16,040	$	(33	)	$	219,783	$	(475	)
Debt securities issued by U.S. government agencies		8		28,890		(13	)		1,000		—			29,890		(13	)
Debt securities issued by the U.S. Treasury		1		10,523		(3	)		—		—			10,523		(3	)
Debt securities issued by states of the United States and political subdivisions of the states		204		65,721		(92	)		18,936		(87	)		84,657		(179	)
Total temporarily impaired securities		381	$	308,877	$	(550	)	$	35,976	$	(120	)	$	344,853	$	(670	)

We believe that the decline in value of these 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 their amortized cost basis at maturity.