4. Investments

Marketable Investments

Held-to-Maturity Investments

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

As of December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Government-sponsored enterprises	$	19.3	$	—	$	—	$	19.3
Corporate notes and bonds		10.8		—		—		10.8

Total	$	30.1	$	—	$	—	$	30.1


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	27.8
Non-current marketable investments		2.3

	$	30.1

As of December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government-sponsored enterprises	$	53.3	$	—	$	(0.2	)	$	53.1
Corporate notes and bonds		106.7		—		—			106.7

Total	$	160.0	$	—	$	(0.2	)	$	159.8


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	122.0
Non-current marketable investments		38.0

	$	160.0

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

	As of December 31,
	2016				2015
	Fair Value		Gross Unrealized Loss		Fair Value		Gross Unrealized Loss
Government-sponsored enterprises:
Continuous unrealized loss position less than one year	$	—	$	—	$	48.1	$	(0.2	)
Continuous unrealized loss position greater than one year		—		—		—		—

		—		—		48.1		(0.2	)
Corporate notes and bonds:
Continuous unrealized loss position less than one year		—		—		63.8		—
Continuous unrealized loss position greater than one year		—		—		—		—

		—		—		63.8		—

Total	$	—	$	—	$	111.9	$	(0.2	)

We attributed the unrealized losses on held-to-maturity securities as of December 31, 2015, to the variability in related market interest rates. We did not intend to sell these securities, nor was it more likely than not that we were required to sell them prior to the end of their contractual terms. Furthermore, we did not believe that these securities exposed us to undue market risk or counterparty credit risk. As such, we did not consider these securities to be other than temporarily impaired.

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

	As of December 31, 2016
	Amortized Cost		Fair Value
Due in less than one year	$	27.8	$	27.8
Due in one to two years		2.3		2.3
Due in three to five years		—		—
Due after five years		—		—

Total	$	30.1	$	30.1

Investments Held at Cost

As of December 31, 2016, we maintain in the aggregate, non-controlling equity investments of approximately $173.2 million in privately-held companies, including a $100.0 million investment in the preferred stock of Synthetic Genomics, Inc. (SGI), which we purchased in two separate $50.0 million transactions in May 2014 and September 2015. These investments are held at cost since we do not have the ability to exercise significant influence over these companies and their fair values are not readily determinable. The fair value of these investments has not been estimated at December 31, 2016, as we have not identified any events or developments indicating that their carrying amounts may be impaired. We include these investments within other non-current assets on our accompanying consolidated balance sheets.

In addition to the SGI investments noted above, we entered into a separate multi-year research and development collaboration agreement with SGI in May 2014, whereby SGI will develop engineered primary pig cells with modified genomes for use in our xenotransplantation program. This collaboration was initially focused primarily on lungs and was expanded in September 2015 to include an additional focus on kidneys. Under this agreement, each party assumes its own research and development costs and SGI may receive royalties and milestone payments from development and commercialization of organs. During the year ended December 31, 2016, we made payments of $36.0 million for investments held at cost.