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

Money market funds of $860.4 million and $1.160 billion at September 30, 2013 and December 31, 2012, 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 September 30, 2013 and December 31, 2012 were as follows:



September 30, 2013	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Estimated Fair Value
U.S. Treasury securities	$	1,017.2	$	0.7	$	—		$	1,017.9
U.S. government-sponsored agency securities	400.1		0.2		(0.9		)	399.4
U.S. government-sponsored agency MBS	587.1		0.5		(7.9		)	579.7
Non-U.S. government, agency and Supranational securities	74.2		—		(0.1		)	74.1
Corporate debt - global	701.8		0.9		(1.2		)	701.5
Asset backed securities	229.6		0.1		(0.3		)	229.4
Marketable equity securities	190.7		231.6		—			422.3
Total available-for-sale marketable securities	$	3,200.7	$	234.0	$	(10.4	)	$	3,424.3



December 31, 2012	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Estimated Fair Value
U.S. Treasury securities	$	902.0	$	0.5	$	—		$	902.5
U.S. government-sponsored agency securities	303.5		0.3		—			303.8
U.S. government-sponsored agency MBS	387.2		1.6		(1.8		)	387.0
Non-U.S. government, agency and Supranational securities	7.1		—		—			7.1
Corporate debt - global	208.5		0.9		(0.2		)	209.2
Marketable equity securities	0.4		—		(0.1		)	0.3
Total available-for-sale marketable securities	$	1,808.7	$	3.3	$	(2.1	)	$	1,809.9

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 mortgage-backed securities (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 equity securities that have become publicly traded. The increase in net unrealized gains in marketable equity securities during the nine-month period ended September 30, 2013 primarily reflects the increase in market value for certain equity investments subsequent to their respective initial public offerings. Net unrealized losses in the marketable debt securities primarily reflect the impact of increased interest rates at September 30, 2013.

Duration periods of available-for-sale debt securities at September 30, 2013 were as follows:



	Amortized Cost		Fair Value
Duration of one year or less	$	1,370.4	$	1,369.3
Duration of one through three years	1,419.0		1,415.7
Duration of three through five years	203.7		200.4
Duration of over five years	16.9		16.6
Total	$	3,010.0	$	3,002.0