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
	Amortized Cost		Gains		Losses		Estimated Fair Value
	(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

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

Gross realized gains and losses from sales of available-for-sale securities are calculated under the specific identification method and are included in investment income. Gross realized investment gains and losses for the years ended December 31, 2016, 2015 and 2014 were insignificant.

We have determined that unrealized gains and losses at December 31, 2016 and 2015 are temporary in nature, because the change in market value for these securities resulted from fluctuating interest rates, rather than a deterioration of the creditworthiness of the issuers. So long as we maintain the intent and ability to hold these securities to maturity, we are unlikely to experience gains or losses. In the event that we dispose of these securities before maturity, we expect that realized gains or losses, if any, will be insignificant.

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	$	—	$	—	—



	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