Short-term Investments

Our short-term investments were as follows:



	March 31, 2016		December 31, 2015
	(in millions)
Available-for-sale investment securities:
U.S. Treasury and agency debt securities	$	598.9	$	527.1
Corporate and other debt securities	394.0		360.5
Certificates of deposit and commercial paper	719.1		754.0
Asset-backed securities	241.9		229.6
Municipal debt securities	121.1		121.3
Mutual funds	22.6		22.3
Total available-for-sale investment securities	2,097.6		2,014.8
Time deposits	830.3		809.5
Total short-term investments	$	2,927.9	$	2,824.3

Our available-for-sale investment securities consist of U.S. dollar denominated investments primarily in U.S. Treasury notes, U.S. government agency debt securities, municipal debt securities, non-U.S. government debt securities, U.S. and international corporate bonds, certificates of deposit, commercial paper, debt securities issued by supranational institutions, mutual funds invested in fixed income securities, and asset-backed securities, including Government National Mortgage Association (GNMA) mortgage backed securities and securities backed by auto loans, credit card receivables, and other receivables. Our investment guidelines are to purchase securities which are investment grade at the time of acquisition. We monitor the credit ratings of the securities in our portfolio on an ongoing basis.

Available-for-Sale Investment Securities

The amortized cost, gross unrealized gains and losses and fair value of available-for-sale investment securities at March 31, 2016 were as follows:



	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in millions)
U.S. Treasury and agency debt securities	$	596.6	$	2.3	$	—		$	598.9
Corporate and other debt securities	392.9		1.3		(0.2		)	394.0
Certificates of deposit and commercial paper	718.6		0.5		—			719.1
Asset-backed securities	241.7		0.4		(0.2		)	241.9
Municipal debt securities	120.6		0.5		—			121.1
Mutual funds	25.7		—		(3.1		)	22.6
Total available-for-sale investment securities	$	2,096.1	$	5.0	$	(3.5	)	$	2,097.6

The amortized cost, gross unrealized gains and losses and fair value of available-for-sale investment securities at December 31, 2015 were as follows:



	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in millions)
U.S. Treasury and agency debt securities	$	528.9	$	—	$	(1.8	)	$	527.1
Corporate and other debt securities	361.9		0.1		(1.5		)	360.5
Certificates of deposit and commercial paper	754.0		0.1		(0.1		)	754.0
Asset-backed securities	230.3		0.1		(0.8		)	229.6
Municipal debt securities	121.2		0.2		(0.1		)	121.3
Mutual funds	25.3		0.1		(3.1		)	22.3
Total available-for-sale investment securities	$	2,021.6	$	0.6	$	(7.4	)	$	2,014.8

The fair value and related unrealized losses of available-for-sale investment securities in a continuous unrealized loss position for less than 12 months and for 12 months or longer were as follows as of March 31, 2016:



	Less than 12 Months					12 Months or More					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(in millions)
U.S. Treasury and agency debt securities	$	43.0	$	—		$	—	$	—		$	43.0	$	—
Corporate and other debt securities	99.0		(0.2		)	3.1		—			102.1		(0.2		)
Asset-backed securities	103.9		(0.1		)	10.5		(0.1		)	114.4		(0.2		)
Municipal debt securities	22.4		—			—		—			22.4		—
Mutual funds	1.1		(0.2		)	21.5		(2.9		)	22.6		(3.1		)
Total	$	269.4	$	(0.5	)	$	35.1	$	(3.0	)	$	304.5	$	(3.5	)



	Less than 12 Months					12 Months or More					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(in millions)
U.S. Treasury and agency debt securities	$	475.7	$	(1.8	)	$	—	$	—		$	475.7	$	(1.8	)
Corporate and other debt securities	315.1		(1.5		)	3.1		—			318.2		(1.5		)
Certificates of deposit and commercial paper	271.5		(0.1		)	—		—			271.5		(0.1		)
Asset-backed securities	199.4		(0.7		)	11.4		(0.1		)	210.8		(0.8		)
Municipal debt securities	56.5		(0.1		)	—		—			56.5		(0.1		)
Mutual funds	—		—			21.1		(3.1		)	21.1		(3.1		)
Total	$	1,318.2	$	(4.2	)	$	35.6	$	(3.2	)	$	1,353.8	$	(7.4	)

The unrealized losses for the above securities as of March 31, 2016 and December 31, 2015 are primarily attributable to changes in interest rates. As of March 31, 2016, we do not consider any of the investments to be other-than-temporarily impaired. The gross unrealized gains and losses in the above tables were recorded, net of tax, in accumulated other comprehensive income (loss).

The contractual maturities of our fixed income available-for-sale investment securities as of March 31, 2016 are set forth in the following table:



	Amortized Cost		Fair Value
	(in millions)
Due within one year	$	815.6	$	816.2
Due after one year up to two years	573.7		575.1
Due after two years up to three years	384.4		386.5
Due after three years up to four years	55.0		55.3
Asset-backed securities	241.7		241.9
Fixed income available-for-sale investment securities	$	2,070.4	$	2,075.0

Asset-backed securities were excluded from the maturity categories because the actual maturities may differ from the contractual maturities since the underlying receivables may be prepaid without penalties. Further, actual maturities of debt securities may differ from those presented above since certain obligations provide the issuer the right to call or prepay the obligation prior to scheduled maturity without penalty.

Proceeds from sales of available-for-sale investment securities and the gross gains and losses that have been included in earnings as a result of those sales were as follows:



	Three Months Ended March 31,
	2016			2015
	(in millions)
Proceeds from sales of available-for-sale investment securities	$	561.6		$	181.8

Gross gains	$	0.3		$	0.4
Gross losses	(0.3		)	(0.1		)
Net realized gains on sales of available-for-sale investment securities	$	—		$	0.3