Financial Instruments

Marketable Securities

The following tables summarize our marketable debt and equity securities:



As of June 30, 2014 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Available-for-sale:
Corporate debt securities
Current	$	125.0	$	0.1	$	—		$	124.9
Non-current	492.2		0.6		(0.2		)	491.8
Government securities
Current	584.9		0.1		(0.1		)	584.9
Non-current	363.1		0.2		—			362.9
Mortgage and other asset backed securities
Current	—		—		—			—
Non-current	155.4		0.1		(0.1		)	155.4
Total marketable debt securities	$	1,720.6	$	1.1	$	(0.4	)	$	1,719.9
Marketable equity securities, non-current	$	0.4	$	—	$	(0.1	)	$	0.5



As of December 31, 2013 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Available-for-sale:
Corporate debt securities
Current	$	100.7	$	—	$	—		$	100.7
Non-current	339.1		0.4		(0.1		)	338.8
Government securities
Current	519.5		—		—			519.5
Non-current	155.2		—		(0.1		)	155.3
Mortgage and other asset backed securities
Current	—		—		—			—
Non-current	131.4		—		(0.1		)	131.5
Total marketable debt securities	$	1,245.9	$	0.4	$	(0.3	)	$	1,245.8
Marketable equity securities, non-current	$	11.2	$	8.7	$	—		$	2.5

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included within cash and cash equivalents on the accompanying condensed consolidated balance sheet:



(In millions)	As of June 30, 2014		As of December 31, 2013
Commercial paper	$	5.9	$	1.2
Overnight reverse repurchase agreements	294.8		22.4
Short-term debt securities	174.5		401.1
Total	$	475.2	$	424.7

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, and our short-term debt securities approximate fair value due to their short term maturities.

Summary of Contractual Maturities: Available-for-Sale Securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:



	As of June 30, 2014				As of December 31, 2013
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	709.8	$	709.8	$	620.2	$	620.2
Due after one year through five years	931.0		930.3		573.1		572.9
Due after five years	79.8		79.8		52.6		52.7
Total available-for-sale securities	$	1,720.6	$	1,719.9	$	1,245.9	$	1,245.8

The average maturity of our marketable debt securities available-for-sale as of June 30, 2014 and December 31, 2013 was 13 months, respectively.

Proceeds from Marketable Debt Securities

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:



	For the Three Months Ended June 30,					For the Six Months Ended June 30,
(In millions)	2014			2013		2014			2013
Proceeds from maturities and sales	$	560.1		$	78.3	$	1,317.5		$	4,404.7
Realized gains	$	0.2		$	—	$	0.4		$	6.3
Realized losses	$	(0.1	)	$	—	$	(0.2	)	$	(2.0	)

Strategic Investments

As of June 30, 2014 and December 31, 2013, our strategic investment portfolio was comprised of investments totaling $51.1 million and $56.9 million, respectively, which are included in investments and other assets in our accompanying condensed consolidated balance sheets.

Our strategic investment portfolio includes investments in marketable equity securities of certain biotechnology companies and our investments in venture capital funds accounted for at fair value which totaled $21.5 million and $33.1 million as of June 30, 2014 and December 31, 2013, respectively. Our strategic investment portfolio also includes other equity investments in privately-held companies and additional investments in venture capital funds accounted for under the cost method. The carrying value of these investments totaled $29.6 million and $23.8 million as of June 30, 2014 and December 31, 2013, respectively.

Changes in Fair Value

During the three and six months ended June 30, 2014 and 2013, we realized changes in fair value recorded through income of $2.0 million and $4.9 million, respectively, on our strategic investment portfolio as compared to $4.4 million and $4.1 million, respectively, in the prior year comparative periods.

Impairments

For the three and six months ended June 30, 2014 and 2013, impairment charges on our marketable equity securities of certain biotechnology companies, investments in venture capital funds accounted for under the cost method and investments in privately-held companies were insignificant.