Investments (Tables)

9 Months Ended

Sep. 30, 2017

Investments, Debt and Equity Securities [Abstract]

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



	September 30, 2017
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,162	$	1	$	1	$	1,162
GSEs	221		—		1		220
Municipal securities	132		—		1		131
Asset-backed securities	125		—		—		125
U.S. treasury notes	121		—		—		121
Certificates of deposit	28		—		—		28
Subtotal - current investments	1,789		1		3		1,787
Corporate debt securities	229		—		—		229
U.S. treasury notes	97		—		—		97
Subtotal - current restricted investments	326		—		—		326
	$	2,115	$	1	$	3	$	2,113



	December 31, 2016
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,180	$	1	$	2	$	1,179
GSEs	232		—		1		231
Municipal securities	143		—		1		142
Asset-backed securities	69		—		—		69
U.S. treasury notes	84		—		—		84
Certificates of deposit	53		—		—		53
	$	1,761	$	1	$	4	$	1,758

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, 2017:



	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	$	783	$	1	314	$	—	$	—	—
GSEs	—		—		—	58		1		20
Municipal securities	97		1		116	—		—		—
	$	880	$	2	430	$	58	$	1	20



	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	$	542	$	2	378	$	—	$	—	—
GSEs	198		1		73	—		—		—
Municipal securities	101		1		129	—		—		—
	$	841	$	4	580	$	—	$	—	—

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 September 30, 2017 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	100	$	100
Due after one year through five years	17		17
	$	117	$	117

The contractual maturities of our available-for-sale investments as of September 30, 2017 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,154	$	1,153
Due after one year through five years	944		943
Due after five years through ten years	17		17
	$	2,115	$	2,113