Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Investments, All Other Investments [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 consolidated balance sheets:

	As of December 31,
(In millions)	2017		2016
Commercial paper	$	30.5	$	31.0
Overnight reverse repurchase agreements	3.6		—
Money market funds	948.0		741.7
Short-term debt securities	247.3		1,266.9
Total	$	1,229.4	$	2,039.6

Marketable securities including strategic investments

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

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

As of December 31, 2016 (In millions)	Fair Value		Gross Unrealized Gains		Gross Unrealized Losses			Amortized Cost
Corporate debt securities
Current	$	1,408.6	$	0.2	$	(0.6	)	$	1,409.0
Non-current	1,255.2		1.2		(4.7		)	1,258.7
Government securities
Current	1,156.0		0.2		(0.3		)	1,156.1
Non-current	1,016.5		0.5		(3.4		)	1,019.4
Mortgage and other asset backed securities
Current	4.0		—		—			4.0
Non-current	557.7		0.8		(2.2		)	559.1
Total marketable debt securities	$	5,398.0	$	2.9	$	(11.2	)	$	5,406.3
Marketable equity securities, non-current	$	24.9	$	0.7	$	(9.3	)	$	33.5

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 December 31, 2017				As of December 31, 2016
(In millions)	Estimated Fair  Value		Amortized Cost		Estimated Fair  Value		Amortized Cost
Due in one year or less	$	2,115.2	$	2,116.0	$	2,568.6	$	2,569.1
Due after one year through five years	2,730.0		2,730.0		2,552.6		2,559.7
Due after five years	327.3		327.0		276.8		277.5
Total available-for-sale securities	$	5,172.5	$	5,173.0	$	5,398.0	$	5,406.3

Proceeds from marketable securities, excluding strategic investments

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:

	For the Years Ended December 31,
(In millions)	2017		2016		2015
Proceeds from maturities and sales	$	5,565.9	$	7,378.9	$	4,063.0
Realized gains	$	3.0	$	3.3	$	1.5
Realized losses	$	22.4	$	4.3	$	3.5