Financial Instruments

Marketable Securities

The following tables summarize our marketable debt and equity securities:



As of June 30, 2015 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Available-for-sale:
Corporate debt securities
Current	$	375.9	$	0.1	$	(0.1	)	$	375.9
Non-current	822.2		0.4		(1.2		)	823.0
Government securities
Current	710.5		0.1		(0.1		)	710.5
Non-current	989.8		0.9		(0.1		)	989.0
Mortgage and other asset backed securities
Current	0.2		—		—			0.2
Non-current	289.8		0.2		(0.3		)	289.9
Total marketable debt securities	$	3,188.4	$	1.7	$	(1.8	)	$	3,188.5
Marketable equity securities, non-current	$	6.8	$	1.1	$	—		$	5.7



As of December 31, 2014 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Available-for-sale:
Corporate debt securities
Current	$	370.4	$	—	$	(0.2	)	$	370.6
Non-current	692.6		0.2		(1.5		)	693.9
Government securities
Current	269.9		—		(0.1		)	270.0
Non-current	579.9		0.3		(0.4		)	580.0
Mortgage and other asset backed securities
Current	0.2		—		—			0.2
Non-current	198.1		0.2		(0.2		)	198.1
Total marketable debt securities	$	2,111.1	$	0.7	$	(2.4	)	$	2,112.8
Marketable equity securities, non-current	$	6.9	$	1.2	$	(0.2	)	$	5.9

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, 2015		As of December 31, 2014
Commercial paper	$	38.8	$	54.2
Overnight reverse repurchase agreements	185.0		305.0
Money market funds	740.2		321.2
Short-term debt securities	10.0		35.9
Total	$	974.0	$	716.3

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and our short-term debt securities approximate fair value due to their short term maturities. Our overnight reverse repurchase agreements are collateralized with agency-guaranteed mortgage securities and represent approximately 1.1% and 2.1% of total assets as of June 30, 2015 and December 31, 2014, respectively.

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, 2015				As of December 31, 2014
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,086.6	$	1,086.6	$	640.5	$	640.8
Due after one year through five years	1,954.4		1,954.5		1,343.7		1,345.2
Due after five years	147.4		147.4		126.9		126.8
Total available-for-sale securities	$	3,188.4	$	3,188.5	$	2,111.1	$	2,112.8

The average maturity of our marketable debt securities available-for-sale as of June 30, 2015 and December 31, 2014 was approximately 17 months and 15 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)	2015			2014			2015			2014
Proceeds from maturities and sales	$	601.9		$	560.1		$	975.5		$	1,317.5
Realized gains	$	0.3		$	0.2		$	0.5		$	0.4
Realized losses	$	(0.5	)	$	(0.1	)	$	(0.8	)	$	(0.2	)

Strategic Investments

As of June 30, 2015 and December 31, 2014, our strategic investment portfolio was comprised of investments totaling $45.3 million and $47.8 million, respectively, which are included in investments and other assets in our accompanying condensed consolidated balance sheets. Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies and investments in venture capital funds where the underlying investments are in equity securities of biotechnology companies.