Fair Value Measurements

We consider the carrying amounts of cash, cash equivalents and other current assets and current liabilities (not including derivatives, the current portion of long-term debt, and certain accounts reported in assets and liabilities held for sale) to approximate their fair values because of the relatively short period of time between the origination of these instruments and their expected realization or payment. For our financial instruments measured at fair value on a recurring basis, we prioritize the inputs used in measuring fair value according to the three-tier fair value hierarchy. For a description of the methods and assumptions that we use to a) estimate the fair value; and b) determine the classification according to the fair value hierarchy for each financial instrument, see Note 4, “Fair Value Measurements,” in our 2017 Annual Report on Form 10-K.

Derivative financial instruments include the 1.125% Call Option derivative asset and the 1.125% Conversion Option derivative liability. These derivatives are not actively traded and are valued based on an option pricing model that uses observable and unobservable market data for inputs. Significant market data inputs used to determine fair value as of June 30, 2018, included the price of our common stock, the time to maturity of the derivative instruments, the risk-free interest rate, and the implied volatility of our common stock. As described further in Note 8, “Derivatives,” the 1.125% Call Option derivative asset and the 1.125% Conversion Option derivative liability were designed such that changes in their fair values would offset, with minimal impact to the consolidated statements of operations. Therefore, the sensitivity of changes in the unobservable inputs to the option pricing model for such derivative instruments is mitigated.

The net changes in fair value of Level 3 financial instruments were insignificant to our results of operations for the six months ended June 30, 2018.

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

Fair Value Measurements – 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.”