Investments (Tables)

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

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



	September 30, 2018
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,197	$	1	$	7	$	1,191
U.S. treasury notes	222		—		1		221
GSEs	172		—		2		170
Municipal securities	121		—		2		119
Asset backed securities	93		—		1		92
Certificates of deposit	15		—		—		15
Other	4		—		—		4
	$	1,824	$	1	$	13	$	1,812



	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

ur held-to-maturity restricted investments are carried at amortized cost, which approximates fair value. Held-to-maturity restricted investments as of September 30, 2018, are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	111	$	111
Due after one year through five years	7		7
	$	118	$	118

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



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,025	$	1,023
Due after one year through five years	799		789
	$	1,824	$	1,812

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 September 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	$	727	$	4	460	$	186	$	3	127
U.S. Treasury notes	—		—		—	94		1		31
GSEs	—		—		—	127		2		68
Municipal securities	63		1		63	55		1		57
Asset backed securities	72		1		41	—		—		—
	$	862	$	6	564	$	462	$	7	283



	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