4. Investments

Marketable Investments

Held-to-Maturity Investments

Marketable investments classified as held-to-maturity consist of the following (in thousands):

As of December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government-sponsored enterprises	$	127,212	$	118	$	(39	)	$	127,291
Corporate notes and bonds		293,288		260		(108	)		293,440

Total	$	420,500	$	378	$	(147	)	$	420,731


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	297,842
Noncurrent marketable investments		122,658

	$	420,500

As of December 31, 2013	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government-sponsored enterprises	$	445,939	$	257	$	(77	)	$	446,119
Corporate notes and bonds		411,455		300		(163	)		411,592

Total	$	857,394	$	557	$	(240	)	$	857,711


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	409,645
Noncurrent marketable investments		447,749

	$	857,394

The following table summarizes gross unrealized losses and the length of time marketable investments have been in a continuous unrealized loss position (in thousands):

	As of December 31,
	2014					2013
	Fair Value		Gross Unrealized Loss			Fair Value		Gross Unrealized Loss
Government-sponsored enterprises:
Continuous unrealized loss position less than one year	$	15,293	$	(39	)	$	76,651	$	(77	)
Continuous unrealized loss position greater than one year		—		—			—		—

		15,293		(39	)		76,651		(77	)
Corporate notes and bonds:
Continuous unrealized loss position less than one year		86,824		(97	)		168,669		(163	)
Continuous unrealized loss position greater than one year		3,443		(11	)		—		—

		90,267		(108	)		168,669		(163	)
Total	$	105,560	$	(147	)	$	245,320	$	(240	)

We attribute the unrealized losses on held-to-maturity securities as of December 31, 2014 and 2013, to the variability in related market interest rates. We do not intend to sell these securities, nor is it more likely than not that we will be required to sell them prior to the end of their contractual terms. Furthermore, we do not believe that these securities expose us to undue market risk or counterparty credit risk. As such, we do not consider these securities to be other than temporarily impaired.

The following table summarizes the contractual maturities of held-to-maturity marketable investments (in thousands):

	As of December 31, 2014
	Amortized Cost		Fair Value
Due in less than one year	$	297,842	$	297,969
Due in one to two years		107,405		107,522
Due in three to five years		15,253		15,240
Due after five years		—		—
Total	$	420,500	$	420,731

Investments Held at Cost

As of December 31, 2014, we maintain in the aggregate, non-controlling equity investments of approximately $83.0 million in privately-held corporations, including a $50.0 million investment in the preferred stock of Synthetic Genomics Inc. (SGI), which we purchased in May 2014. We account for these investments under the cost method since we do not have the ability to exercise significant influence over these companies and their fair values are not readily determinable. The fair values of these investments have not been estimated at December 31, 2014, as we did not identify any events or developments indicating that their carrying amounts may be impaired. We include these investments within other assets on our accompanying consolidated balance sheets.

In addition to the SGI investment noted above, we entered into a separate multi-year research and development collaboration agreement whereby SGI will develop engineered primary pig cells with modified genomes for use in our xenotransplantation program, which is primarily focused on lungs. Under this agreement, each party will assume its own research and development costs and SGI may receive royalties and milestone payments from the development and commercialization of organs.