Cash, Cash Equivalents and Marketable Securities Available-for-Sale

Money market funds of $1.413 billion and $2.251 billion at December 31, 2015 and 2014, respectively, were recorded at cost, which approximates fair value and are included in cash and cash equivalents.

The amortized cost, gross unrealized holding gains, gross unrealized holding losses and estimated fair value of available-for-sale securities by major security type and class of security at December 31, 2015 and 2014 were as follows:



December 31, 2015	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Estimated Fair Value
U.S. Treasury securities	$	153.0	$	—	$	(0.4	)	$	152.6
U.S. government-sponsored agency MBS	29.8		0.1		(0.4		)	29.5
Corporate debt – global	219.7		—		(1.6		)	218.1
Asset backed securities	35.6		—		(0.1		)	35.5
Marketable equity securities	811.5		468.1		(43.7		)	1,235.9
Total available-for-sale marketable securities	$	1,249.6	$	468.2	$	(46.2	)	$	1,671.6



December 31, 2014	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Estimated Fair Value
U.S. Treasury securities	$	1,044.7	$	0.3	$	(0.8	)	$	1,044.2
U.S. government-sponsored agency securities	145.1		0.1		(0.1		)	145.1
U.S. government-sponsored agency MBS	531.1		1.0		(2.7		)	529.4
Non-U.S. government, agency and supranational securities	32.4		—		(0.1		)	32.3
Corporate debt – global	446.3		0.6		(1.2		)	445.7
Asset backed securities	177.3		—		(0.2		)	177.1
Marketable equity securities	335.2		716.3		(0.2		)	1,051.3
Total available-for-sale marketable securities	$	2,712.1	$	718.3	$	(5.3	)	$	3,425.1

U.S. government-sponsored agency securities include general unsecured obligations either issued directly by or guaranteed by U.S. Government Sponsored Enterprises. U.S. government-sponsored agency MBS include mortgage-backed securities issued by the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation and the Government National Mortgage Association. Non-U.S. government, agency and supranational securities consist of direct obligations of highly rated governments of nations other than the United States and obligations of sponsored agencies and other entities that are guaranteed or supported by highly rated governments of nations other than the United States. Corporate debt-global includes obligations issued by investment-grade corporations, including some issues that have been guaranteed by governments and government agencies. Asset backed securities consist of triple-A rated securities with cash flows collateralized by credit card receivables and auto loans. Marketable equity securities consist of investments in publicly traded equity securities. The decrease in net unrealized gains in marketable equity securities in 2015 compared to 2014 primarily reflects the decrease in market value for certain equity investments subsequent to their respective initial public offerings. Net unrealized losses in marketable debt securities primarily reflect the impact of increased interest rates at December 31, 2015.

The fair value of all available-for-sale securities, which have been in an unrealized loss position for less than and longer than 12 months at December 31, 2015, was as follows:



	Less than 12 months					12 months or longer					Total
December 31, 2015	Estimated Fair Value		Gross Unrealized Loss			Estimated Fair Value		Gross Unrealized Loss			Estimated Fair Value		Gross Unrealized Loss
U.S. Treasury securities	$	143.6	$	(0.4	)	$	—	$	—		$	143.6	$	(0.4	)
U.S. government-sponsored agency MBS	12.8		(0.3		)	6.8		(0.1		)	19.6		(0.4		)
Corporate debt – global	193.5		(1.6		)	—		—			193.5		(1.6		)
Asset backed securities	29.6		(0.1		)	5.9		—			35.5		(0.1		)
Marketable equity securities	494.4		(43.7		)	—		—			494.4		(43.7		)
Total	$	873.9	$	(46.1	)	$	12.7	$	(0.1	)	$	886.6	$	(46.2	)

The Company believes that the decline in fair value of securities held at December 31, 2015 below their cost is temporary and intends to retain its investment in these securities for a sufficient period of time to allow for recovery in the market value of these investments.

Duration periods of available-for-sale debt securities at December 31, 2015 were as follows:



	Amortized Cost		Fair Value
Duration of one year or less	$	58.4	$	58.3
Duration of one through three years	363.7		361.8
Duration of three through five years	15.9		15.6
Total	$	438.0	$	435.7