INVESTMENT SECURITIES

Investment securities classified as current and long-term were as follows at December 31, 2018 and 2017, respectively:



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in millions)
December 31, 2018
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	419	$	1	$	(3	)	$	417
Mortgage-backed securities	2,595		3		(54		)	2,544
Tax-exempt municipal securities	2,805		3		(37		)	2,771
Mortgage-backed securities:
Residential	55		—		—			55
Commercial	537		—		(14		)	523
Asset-backed securities	991		1		(7		)	985
Corporate debt securities	3,239		1		(98		)	3,142
Total debt securities	$	10,641	$	9	$	(213	)	$	10,437
December 31, 2017
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	532	$	1	$	(2	)	$	531
Mortgage-backed securities	1,625		4		(19		)	1,610
Tax-exempt municipal securities	3,884		33		(28		)	3,889
Mortgage-backed securities:
Residential	26		—		—			26
Commercial	455		3		(2		)	456
Asset-backed securities	407		1		—			408
Corporate debt securities	5,175		244		(37		)	5,382
Total debt securities	$	12,104	$	286	$	(88	)	$	12,302

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 December 31, 2018 and 2017, 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
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
	(in millions)
December 31, 2018
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	179	$	(1	)	$	153	$	(2	)	$	332	$	(3	)
Mortgage-backed securities	956		(16		)	1,019		(38		)	1,975		(54		)
Tax-exempt municipal securities	809		(9		)	1,648		(28		)	2,457		(37		)
Mortgage-backed securities:
Residential	—		—			15		—			15		—
Commercial	372		(8		)	133		(6		)	505		(14		)
Asset-backed securities	824		(7		)	40		—			864		(7		)
Corporate debt securities	1,434		(35		)	1,439		(63		)	2,873		(98		)
Total debt securities	$	4,574	$	(76	)	$	4,447	$	(137	)	$	9,021	$	(213	)
December 31, 2017
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	273	$	(1	)	$	130	$	(1	)	$	403	$	(2	)
Mortgage-backed securities	581		(2		)	672		(17		)	1,253		(19		)
Tax-exempt municipal securities	1,590		(16		)	661		(12		)	2,251		(28		)
Mortgage-backed securities:
Residential	20		—			3		—			23		—
Commercial	131		(1		)	28		(1		)	159		(2		)
Asset-backed securities	107		—			10		—			117		—
Corporate debt securities	1,297		(10		)	804		(27		)	2,101		(37		)
Total debt securities	$	3,999	$	(30	)	$	2,308	$	(58	)	$	6,307	$	(88	)

Approximately 97% of our debt securities were investment-grade quality, with a weighted average credit rating of AA by S&P at December 31, 2018. 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%. Our investment policy limits investments in a single issuer and requires diversification among various asset types.

Our unrealized loss from all securities was generated from approximately 1,210 positions out of a total of approximately 1,500 positions at December 31, 2018. All issuers of securities we own that were trading at an unrealized loss at December 31, 2018 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 December 31, 2018, 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 December 31, 2018.

The detail of realized gains (losses) related to investment securities and included within investment income was as follows for the years ended December 31, 2018, 2017, and 2016:



	2018			2017			2016
	(in millions)
Gross realized gains	$	106		$	35		$	120
Gross realized losses	(16		)	(21		)	(24		)
Net realized capital gains	$	90		$	14		$	96

There were no material other-than-temporary impairments in 2018, 2017, or 2016.

The contractual maturities of debt securities available for sale at December 31, 2018, 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	$	943	$	941
Due after one year through five years	2,929		2,873
Due after five years through ten years	1,873		1,810
Due after ten years	718		706
Mortgage and asset-backed securities	4,178		4,107
Total debt securities	$	10,641	$	10,437