INVESTMENT SECURITIES

Investment securities classified as current and long-term were as follows at September 30, 2017 and December 31, 2016, respectively:



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in millions)
September 30, 2017
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	821	$	1	$	(8	)	$	814
Mortgage-backed securities	1,423		5		(19		)	1,409
Tax-exempt municipal securities	3,457		28		(16		)	3,469
Mortgage-backed securities:
Residential	7		—		—			7
Commercial	399		3		(2		)	400
Asset-backed securities	140		—		—			140
Corporate debt securities	4,921		215		(37		)	5,099
Total debt securities	$	11,168	$	252	$	(82	)	$	11,338

December 31, 2016
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	800	$	1	$	(15	)	$	786
Mortgage-backed securities	1,662		6		(31		)	1,637
Tax-exempt municipal securities	3,358		15		(68		)	3,305
Mortgage-backed securities:
Residential	9		—		—			9
Commercial	307		1		(4		)	304
Asset-backed securities	160		—		—			160
Corporate debt securities	3,530		145		(78		)	3,597
Total debt securities	$	9,826	$	168	$	(196	)	$	9,798

Gross unrealized losses and fair values aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position were as follows at September 30, 2017 and December 31, 2016, respectively:



	Less than 12 months					12 months or more					Total
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
	(in millions)
September 30, 2017
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	522	$	(5	)	$	137	$	(3	)	$	659	$	(8	)
Mortgage-backed securities	986		(17		)	82		(2		)	1,068		(19		)
Tax-exempt municipal securities	1,320		(9		)	460		(7		)	1,780		(16		)
Mortgage-backed securities:
Residential	—		—			4		—			4		—
Commercial	115		(2		)	—		—			115		(2		)
Asset-backed securities	64		—			—		—			64		—
Corporate debt securities	999		(15		)	469		(22		)	1,468		(37		)
Total debt securities	$	4,006	$	(48	)	$	1,152	$	(34	)	$	5,158	$	(82	)

December 31, 2016
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	697	$	(15	)	$	3	$	—		$	700	$	(15	)
Mortgage-backed securities	1,528		(31		)	3		—			1,531		(31		)
Tax-exempt municipal securities	2,756		(67		)	43		(1		)	2,799		(68		)
Mortgage-backed securities:
Residential	—		—			4		—			4		—
Commercial	182		(3		)	24		(1		)	206		(4		)
Asset-backed securities	51		—			63		—			114		—
Corporate debt securities	1,544		(71		)	69		(7		)	1,613		(78		)
Total debt securities	$	6,758	$	(187	)	$	209	$	(9	)	$	6,967	$	(196	)

Approximately 98% of our debt securities were investment-grade quality, with a weighted average credit rating of AA by Standard & Poor's Rating Service, or S&P, at September 30, 2017. Most of the debt securities that were below investment-grade were rated BB, the higher end of the below investment-grade rating scale. Tax-exempt municipal securities were diversified among general obligation bonds of states and local municipalities in the United States as well as special revenue bonds issued by municipalities to finance specific public works projects such as utilities, water and sewer, transportation, or education. Our general obligation bonds are diversified across the United States with no individual state exceeding 9%. In addition, 2% of our tax-exempt securities were insured by bond insurers and had an equivalent weighted average S&P credit rating of AA exclusive of the bond insurers’ guarantee. Our investment policy limits investments in a single issuer and requires diversification among various asset types.

Our unrealized losses from all securities were generated from approximately 680 positions out of a total of approximately 2,320 positions at September 30, 2017. All issuers of securities we own that were trading at an unrealized loss at September 30, 2017 remain current on all contractual payments. After taking into account these and other factors previously described, we believe these unrealized losses primarily were caused by an increase in market interest rates in the current markets since the time the securities were purchased. At September 30, 2017, we did not intend to sell the securities with an unrealized loss position in accumulated other comprehensive income, and it is not likely that we will be required to sell these securities before recovery of their amortized cost basis. As a result, we believe that the securities with an unrealized loss were not other-than-temporarily impaired at September 30, 2017.

The detail of realized gains (losses) related to investment securities and included within investment income was as follows for the three and nine months ended September 30, 2017 and 2016:



	Three months ended September 30,						Nine months ended September 30,
	2017			2016			2017			2016
	(in millions)
Gross realized gains	$	3		$	37		$	34		$	88
Gross realized losses	(3		)	(11		)	(6		)	(23		)
Net realized capital gains	$	—		$	26		$	28		$	65

There were no material other-than-temporary impairments for the three and nine months ended September 30, 2017 or 2016.

The contractual maturities of debt securities available for sale at September 30, 2017, regardless of their balance sheet classification, are shown below. Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.



	Amortized Cost		Fair Value
	(in millions)
Due within one year	$	501	$	502
Due after one year through five years	2,999		3,014
Due after five years through ten years	2,555		2,561
Due after ten years	3,144		3,305
Mortgage and asset-backed securities	1,969		1,956
Total debt securities	$	11,168	$	11,338