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

Money market funds of $1.599 billion and $1.413 billion at June 30, 2016 and December 31, 2015, 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 June 30, 2016 and December 31, 2015 were as follows:



June 30, 2016	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Estimated Fair Value
U.S. Treasury securities	$	124.6	$	0.4	$	—		$	125.0
U.S. government-sponsored agency MBS	28.6		0.1		—			28.7
Corporate debt - global	163.9		1.1		(0.1		)	164.9
Asset backed securities	26.5		0.1		—			26.6
Marketable equity securities	887.2		213.6		(106.0		)	994.8
Total available-for-sale marketable securities	$	1,230.8	$	215.3	$	(106.1	)	$	1,340.0



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

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. 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 during the six-month period ended June 30, 2016 primarily reflects the decrease in market value for certain equity investments subsequent to December 31, 2015.

Duration periods of available-for-sale debt securities at June 30, 2016 were as follows:



	Amortized Cost		Fair Value
Duration of one year or less	$	41.4	$	41.4
Duration of one through three years	288.4		289.8
Duration of three through five years	13.8		14.0
Total	$	343.6	$	345.2