Financial Instruments

Marketable Securities

The following tables summarize our marketable debt and equity securities:



As of March 31, 2015 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Available-for-sale:
Corporate debt securities
Current	$	385.3	$	0.1	$	(0.1	)	$	385.3
Non-current	605.7		0.8		(0.4		)	605.3
Government securities
Current	288.5		—		(0.1		)	288.6
Non-current	562.1		0.6		(0.1		)	561.6
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	209.5		0.4		(0.1		)	209.2
Total marketable debt securities	$	2,051.2	$	1.9	$	(0.8	)	$	2,050.1
Marketable equity securities, non-current	$	6.2	$	0.3	$	—		$	5.9



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 March 31, 2015		As of December 31, 2014
Commercial paper	$	10.3	$	54.2
Overnight reverse repurchase agreements	308.6		305.0
Money market funds	427.2		321.2
Short-term debt securities	6.4		35.9
Total	$	752.5	$	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.

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 March 31, 2015				As of December 31, 2014
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	673.9	$	674.0	$	640.5	$	640.8
Due after one year through five years	1,252.0		1,251.0		1,343.7		1,345.2
Due after five years	125.3		125.1		126.9		126.8
Total available-for-sale securities	$	2,051.2	$	2,050.1	$	2,111.1	$	2,112.8

The average maturity of our marketable debt securities available-for-sale as of March 31, 2015 and December 31, 2014 was approximately 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 March 31,
(In millions)	2015			2014
Proceeds from maturities and sales	$	373.6		$	757.5
Realized gains	$	0.2		$	0.2
Realized losses	$	(0.3	)	$	(0.1	)

Strategic Investments

As of March 31, 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 marketable equity securities of certain biotechnology companies and our investments in venture capital funds accounted for at fair value which totaled $18.3 million and $21.4 million as of March 31, 2015 and December 31, 2014, 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 $27.0 million and $26.4 million as of March 31, 2015 and December 31, 2014, respectively.