Financial Instruments

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



	As of December 31,
(In millions)	2015		2014
Commercial paper	$	21.9	$	54.2
Overnight reverse repurchase agreements	134.7		305.0
Money market funds	673.8		321.2
Short-term debt securities	79.1		35.9
Total	$	909.5	$	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. Our overnight reverse repurchase agreements are collateralized with agency-guaranteed mortgage-backed securities and represent approximately 0.7% and 2.1% of total assets as of December 31, 2015 and 2014, respectively.

The following tables summarize our marketable debt and equity securities:



As of December 31, 2015 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	394.3	$	—	$	(0.5	)	$	394.8
Non-current	1,116.6		0.1		(4.1		)	1,120.6
Government securities
Current	1,723.4		0.1		(1.1		)	1,724.4
Non-current	1,152.5		—		(3.1		)	1,155.6
Mortgage and other asset backed securities
Current	2.8		—		—			2.8
Non-current	491.3		0.1		(1.8		)	493.0
Total marketable debt securities	$	4,880.9	$	0.3	$	(10.6	)	$	4,891.2
Marketable equity securities, non-current	$	37.5	$	9.2	$	—		$	28.3



As of December 31, 2014 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
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

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 December 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	$	2,120.5	$	2,122.0	$	640.5	$	640.8
Due after one year through five years	2,575.9		2,583.9		1,343.7		1,345.2
Due after five years	184.5		185.3		126.9		126.8
Total available-for-sale securities	$	4,880.9	$	4,891.2	$	2,111.1	$	2,112.8

The average maturity of our marketable debt securities available-for-sale as of December 31, 2015 and 2014, was 16 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 Years Ended December 31,
(In millions)	2015		2014		2013
Proceeds from maturities and sales	$	4,063.0	$	2,718.9	$	5,190.1
Realized gains	$	1.5	$	0.7	$	6.6
Realized losses	$	3.5	$	0.5	$	2.1

Realized losses for the year ended December 31, 2015, primarily relate to sales of corporate bonds, agency mortgage-backed securities and other asset-backed securities. Realized losses for the year ended December 31, 2014, primarily relate to sales of agency mortgage-backed securities and government securities. Realized losses for the year ended December 31, 2013, primarily relate to sales of agency mortgage-backed securities and corporate securities.

Strategic Investments

As of December 31, 2015 and 2014, our strategic investment portfolio was comprised of investments totaling $96.0 million and $47.8 million, respectively, which are included in investments and other assets in our 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.