Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

Fair value of assets measured on recurring basis

Our financial instruments measured at fair value on a recurring basis at June 30, 2018, were as follows:



	Total		Quoted Market Prices (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(In millions)
Corporate debt securities	$	1,382	$	—	$	1,382	$	—
U.S. treasury notes	330		330		—		—
Government-sponsored enterprise securities (GSEs)	207		207		—		—
Municipal securities	136		—		136		—
Asset-backed securities	99		—		99		—
Certificate of deposit	19		—		19		—
Other	3		—		3		—
Subtotal - current investments	2,176		537		1,639		—
Corporate debt securities	55		—		55		—
U.S. treasury notes	25		25		—		—
Subtotal - current restricted investments	80		25		55		—
1.125% Call Option derivative asset	657		—		—		657
Total assets	$	2,913	$	562	$	1,694	$	657

1.125% Conversion Option derivative liability	$	657	$	—	$	—	$	657
Total liabilities	$	657	$	—	$	—	$	657

Our financial instruments measured at fair value on a recurring basis at December 31, 2017, were as follows:



	Total		Quoted Market Prices (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(In millions)
Corporate debt securities	$	1,588	$	—	$	1,588	$	—
U.S. treasury notes	388		388		—		—
GSEs	253		253		—		—
Municipal securities	141		—		141		—
Asset-backed securities	117		—		117		—
Certificates of deposit	37		—		37		—
Subtotal - current investments	2,524		641		1,883		—
Corporate debt securities	101		—		101		—
U.S. treasury notes	68		68		—		—
Subtotal - current restricted investments	169		68		101		—
1.125% Call Option derivative asset	522		—		—		522
Total assets	$	3,215	$	709	$	1,984	$	522

1.125% Conversion Option derivative liability	$	522	$	—	$	—	$	522
Total liabilities	$	522	$	—	$	—	$	522

Schedule of fair value, asset and liabilities measured on recurring basis - disclosure only

The carrying amounts and estimated fair values of our senior notes are classified as Level 2 financial instruments. Fair value for these securities is determined using a market approach based on quoted market prices for similar securities in active markets or quoted prices for identical securities in inactive markets. The carrying amount and estimated fair value of the amount due under our Credit Facility was classified as a Level 3 financial instrument, because certain inputs used to determine its fair value were not observable. The carrying amount of the amount due under the Credit Facility as of December 31, 2017, approximated its fair value because the Credit Facility’s interest rate is a variable rate that approximates rates currently available to us.



	June 30, 2018				December 31, 2017
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(In millions)
5.375% Notes	$	693	$	706	$	692	$	730
1.125% Convertible Notes ⁽¹⁾	420		1,099		496		1,052
4.875% Notes	326		320		325		329
1.625% Convertible Notes	63		107		157		220
Credit Facility	—		—		300		300
	$	1,502	$	2,232	$	1,970	$	2,631

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(1)	The fair value of the 1.125% Conversion Option (the embedded cash conversion option) amounted to $657 million and $522 million as of June 30, 2018, and December 31, 2017, respectively. See further discussion at Note 7, “Debt,” and Note 8, “Derivatives.”