Investments (Tables)

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Investments

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

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

	December 31, 2015
	Amortized Cost		Gross Unrealized				Estimated Fair Value
			Gains		Losses
	(In millions)
Corporate debt securities	$	1,189	$	—	$	5	$	1,184
GSEs	212		—		1		211
Municipal securities	186		—		1		185
U.S. treasury notes	78		—		—		78
Asset-backed securities	63		—		—		63
Certificates of deposit	80		—		—		80
	$	1,808	$	—	$	7	$	1,801

Contractual maturities of investments

The contractual maturities of our investments as of December 31, 2016 are summarized below:

	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	930	$	930
Due after one year through five years	806		803
Due after five years through ten years	25		25
	$	1,761	$	1,758

Available-for-sale investments

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 loss position for 12 months or more as of December 31, 2016.

	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	$	—	$	—	—

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 loss position for 12 months or more as of December 31, 2015.

	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	$	825	$	4	588	$	119	$	1	87
GSEs	182		1		77	—		—		—
Municipal securities	128		1		181	—		—		—
	$	1,135	$	6	846	$	119	$	1	87