Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Summary of financial assets with maturities of less than 90 days included within cash and cash equivalents

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 in our condensed consolidated balance sheets:

(In millions)	As of March 31, 2018		As of December 31, 2017
Commercial paper	$	51.5	$	30.5
Overnight reverse repurchase agreements	—		3.6
Money market funds	3,561.2		948.0
Short-term debt securities	161.0		247.3
Total	$	3,773.7	$	1,229.4

Marketable debt and equity securities

The following tables summarize our marketable debt and equity securities, classified as available-for-sale:

As of March 31, 2018 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	912.9	$	—	$	(0.8	)	$	913.7
Non-current	645.0		0.3		(0.7		)	645.4
Government securities
Current	895.0		—		(0.5		)	895.5
Non-current	248.2		0.1		(0.3		)	248.4
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	307.0		0.1		(1.0		)	307.9
Total marketable debt securities	$	3,008.2	$	0.5	$	(3.3	)	$	3,011.0

As of December 31, 2017 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,039.3	$	—	$	(0.2	)	$	1,039.5
Non-current	1,570.5		0.9		—			1,569.6
Government securities
Current	1,075.1		0.1		(0.7		)	1,075.7
Non-current	844.2		0.2		(1.1		)	845.1
Mortgage and other asset backed securities
Current	0.8		—		—			0.8
Non-current	642.6		1.1		(0.8		)	642.3
Total marketable debt securities	$	5,172.5	$	2.3	$	(2.8	)	$	5,173.0
Marketable equity securities, non-current	$	11.8	$	1.8	$	(4.4	)	$	14.4

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 March 31, 2018				As of December 31, 2017
(In millions)	Estimated Fair Value		Amortized Cost		Estimated Fair Value		Amortized Cost
Due in one year or less	$	1,808.0	$	1,809.3	$	2,115.2	$	2,116.0
Due after one year through five years	1,123.0		1,124.2		2,730.0		2,730.0
Due after five years	77.2		77.5		327.3		327.0
Total available-for-sale securities	$	3,008.2	$	3,011.0	$	5,172.5	$	5,173.0

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 Three Months Ended March 31,
(In millions)	2018			2017
Proceeds from maturities and sales	$	4,068.9		$	1,884.3
Realized gains	$	1.8		$	1.2
Realized losses	$	(9.4	)	$	(1.9	)