Investments (Tables)

6 Months Ended

Jun. 30, 2018

Investments, Debt and Equity Securities [Abstract]

The following tables summarize our investments as of the dates indicated:



	June 30, 2018
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,390	$	—	$	8	$	1,382
U.S. treasury notes	331		—		1		330
GSEs	209		—		2		207
Municipal securities	138		—		2		136
Asset backed securities	100		—		1		99
Certificates of deposit	19		—		—		19
Other	3		—		—		3
Subtotal - current investments	2,190		—		14		2,176
Corporate debt securities	55		—		—		55
U.S. treasury notes	25		—		—		25
Subtotal - current restricted investments	80		—		—		80
	$	2,270	$	—	$	14	$	2,256



	December 31, 2017
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,591	$	1	$	4	$	1,588
U.S. treasury notes	389		—		1		388
GSEs	255		—		2		253
Municipal securities	142		—		1		141
Asset-backed securities	117		—		—		117
Certificates of deposit	37		—		—		37
Subtotal - current investments	2,531		1		8		2,524
Corporate debt securities	101		—		—		101
U.S. treasury notes	68		—		—		68
Subtotal - current restricted investments	169		—		—		169
	$	2,700	$	1	$	8	$	2,693

Contractual maturities of investments

June 30, 2018, and mature in one year or less.

The contractual maturities of our available-for-sale investments as of June 30, 2018 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,356	$	1,354
Due after one year through five years	914		902
	$	2,270	$	2,256

Schedule of available for sale securities continuous unrealized loss position

The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of June 30, 2018:



	In a Continuous Loss Position for Less than 12 Months					In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value		Unrealized Losses		Total Number of Positions	Estimated Fair Value		Unrealized Losses		Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	959	$	7	535	$	116	$	1	72
U.S. Treasury notes	224		1		52	—		—		—
GSEs	—		—		—	113		2		56
Municipal securities	81		1		79	40		1		52
Asset backed securities	90		1		59	—		—		—
	$	1,354	$	10	725	$	269	$	4	180



	In a Continuous Loss Position for Less than 12 Months					In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value		Unrealized Losses		Total Number of Positions	Estimated Fair Value		Unrealized Losses		Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	1,297	$	3	561	$	94	$	1	69
U.S. Treasury Notes	470		1		89	—		—		—
GSEs	173		1		69	95		1		47
Municipal securities	—		—		—	38		1		48
	$	1,940	$	5	719	$	227	$	3	164