Investments (Tables)

3 Months Ended

Mar. 31, 2017

Investments, Debt and Equity Securities [Abstract]

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



	March 31, 2017
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,354	$	1	$	2	$	1,353
GSEs	210		—		1		209
U.S. treasury notes	193		—		—		193
Municipal securities	153		—		1		152
Asset-backed securities	110		—		—		110
Certificates of deposit	39		—		—		39
	$	2,059	$	1	$	4	$	2,056



	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
U.S. treasury notes	84		—		—		84
Municipal securities	143		—		1		142
Asset-backed securities	69		—		—		69
Certificates of deposit	53		—		—		53
	$	1,761	$	1	$	4	$	1,758

Contractual maturities of investments

The contractual maturities of our investments as of March 31, 2017 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,151	$	1,151
Due after one year through five years	872		870
Due after five years through ten years	36		35
	$	2,059	$	2,056

The contractual maturities of our restricted investments, which are designated as held-to-maturity and are carried at amortized cost, which approximates fair value, as of March 31, 2017 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	97	$	97
Due after one year through five years	18		18
	$	115	$	115

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 loss position for 12 months or more as of March 31, 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	$	570	$	2	341	$	—	$	—	—
GSEs	200		1		83	—		—		—
Municipal securities	84		1		101	—		—		—
	$	854	$	4	525	$	—	$	—	—



	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	$	—	$	—	—