Investments

As of June 30, 2015, we have 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 June 30, 2015:

One year or less	54%
After one year but within two years	29%
After two years but within three and a half years	17%
Total	100%

As illustrated above, at June 30, 2015, 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 June 30, 2015, 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							Other- Than- Temporary Impairment				Estimated
June 30, 2015	Cost			Gains			Losses				Loss				Fair Value
Available-for-sale securities (1):
Corporate debt securities		$	189,487		$	37		$	(90	)		$	—			$	189,434
Debt securities issued by U.S. government agencies			81,639			5			(11	)			—				81,633
Debt securities issued by states of the United States and political subdivisions of the states (2)			59,835			22			(58	)			—				59,799
Total securities with a maturity of one year or less			330,961			64			(159	)			—				330,866
Corporate debt securities			234,088			52			(848	)			—				233,292
Debt securities issued by U.S. government agencies			28,002			10			(3	)			—				28,009
Debt securities issued by states of the United States and political subdivisions of the states			62,203			32			(189	)			—				62,046
Total securities with a maturity of more than one year			324,293			94			(1,040	)			—				323,347
Total available-for-sale securities		$	655,254		$	158		$	(1,199	)		$	—			$	654,213
Equity securities:
Regulus Therapeutics Inc.		$	12,477		$	48,127		$	—			$	—			$	60,604
Securities included in other current assets			880			—			—				(880	)			—
Total equity securities		$	13,357		$	48,127		$	—			$	(880	)		$	60,604
Total available-for-sale and equity securities		$	668,611		$	48,285		$	(1,199	)		$	(880	)		$	714,817

				Gross Unrealized							Other- Than- Temporary Impairment				Estimated
December 31, 2014	Cost			Gains			Losses				Loss				Fair Value
Available-for-sale securities (1):
Corporate debt securities (2)		$	219,856		$	89		$	(89	)		$	—			$	219,856
Debt securities issued by U.S. government agencies			47,496			7			(27	)			—				47,476
Debt securities issued by the U.S. Treasury (2)			19,008			9			—				—				19,017
Debt securities issued by states of the United States and political subdivisions of the states (2)			45,196			19			(53	)			—				45,162
Total securities with a maturity of one year or less			331,556			124			(169	)			—				331,511
Corporate debt securities			152,730			16			(600	)			—				152,146
Debt securities issued by U.S. government agencies			62,530			—			(151	)			—				62,379
Debt securities issued by states of the United States and political subdivisions of the states			60,073			32			(234	)			—				59,871
Total securities with a maturity of more than one year			275,333			48			(985	)			—				274,396
Total available-for-sale securities		$	606,889		$	172		$	(1,154	)		$	—			$	605,907
Equity securities:
Regulus Therapeutics Inc.		$	12,477		$	69,404		$	—			$	—			$	81,881
Securities included in other current assets			880			—			—				(880	)			—
Total equity securities		$	13,357		$	69,404		$	—			$	(880	)		$	81,881
Total available-for-sale and equity securities		$	620,246		$	69,576		$	(1,154	)		$	(880	)		$	687,788

(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 considered to be temporarily impaired at June 30, 2015 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		254	$	296,493	$	(894	)	$	9,263	$	(44	)	$	305,756	$	(938	)
Debt securities issued by U.S. government agencies		12		62,116		(14	)		—		—			62,116		(14	)
Debt securities issued by states of the United States and political subdivisions of the states		49		51,616		(178	)		6,229		(69	)		57,845		(247	)
Total temporarily impaired securities		315	$	410,225	$	(1,086	)	$	15,492	$	(113	)	$	425,717	$	(1,199	)

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.