Investments (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

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



	March 31, 2018
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,627	$	—	$	9	$	1,618
U.S. treasury notes	330		—		1		329
GSEs	229		—		2		227
Municipal securities	133		—		2		131
Asset backed securities	111		—		1		110
Certificates of deposit	27		—		—		27
Other	2		—		—		2
Subtotal - current investments	2,459		—		15		2,444
Corporate debt securities	66		—		—		66
U.S. treasury notes	11		—		—		11
Subtotal - current restricted investments	77		—		—		77
	$	2,536	$	—	$	15	$	2,521



	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

The contractual maturities of our held-to-maturity restricted investments, which are carried at amortized cost, which approximates fair value, as of March 31, 2018, are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	117	$	117
Due after one year through five years	3		3
	$	120	$	120

The contractual maturities of our available-for-sale investments as of March 31, 2018 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,576	$	1,573
Due after one year through five years	959		947
Due after five years through ten years	1		1
	$	2,536	$	2,521

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 March 31, 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	$	1,276	$	7	610	$	93	$	2	71
U.S. Treasury notes	364		1		81	—		—		—
GSEs	179		1		67	95		1		47
Municipal securities	85		1		97	38		1		46
Asset backed securities	97		1		60	—		—		—
	$	2,001	$	11	915	$	226	$	4	164



	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