Investments

Available-for-Sale Investments

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

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

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 three months ended March 31, 2017 and 2016 were insignificant.

We have determined that unrealized losses at March 31, 2017 and December 31, 2016, are temporary in nature, because the change in market value for these securities has resulted from fluctuating interest rates, rather than a deterioration of the credit worthiness of the issuers. So long as we maintain the intent and ability to hold these securities to maturity, we are unlikely to experience losses. In the event that we dispose of these securities before maturity, we expect that realized 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 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	$	—	$	—	—

Held-to-Maturity Investments

Pursuant to the regulations governing our Health Plans segment subsidiaries, we maintain statutory deposits and deposits required by government authorities primarily in certificates of deposit and U.S. treasury securities. We also maintain restricted investments as protection against the insolvency of certain capitated providers. The use of these funds is limited as required by regulation in the various states in which we operate, or as needed in the event of insolvency of capitated providers. We have the ability to hold our restricted investments until maturity, and as a result, we would not expect the value of these investments to decline significantly due to a sudden change in market interest rates.

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