INVESTMENT SECURITIES

Investment securities classified as current and long-term were as follows at March 31, 2019 and December 31, 2018, respectively:



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in millions)
March 31, 2019
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	478	$	2	$	(1	)	$	479
Mortgage-backed securities	3,142		21		(22		)	3,141
Tax-exempt municipal securities	2,096		12		(5		)	2,103
Mortgage-backed securities:
Residential	84		1		—			85
Commercial	510		2		(3		)	509
Asset-backed securities	1,037		1		(6		)	1,032
Corporate debt securities	2,959		14		(24		)	2,949
Total debt securities	$	10,306	$	53	$	(61	)	$	10,298

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

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 March 31, 2019 and December 31, 2018, 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)
March 31, 2019
U.S. Treasury and other U.S. government corporations and agencies:
U.S. Treasury and agency obligations	$	25	$	—		$	214	$	(1	)	$	239	$	(1	)
Mortgage-backed securities	255		(1		)	1,152		(21		)	1,407		(22		)
Tax-exempt municipal securities	30		—			924		(5		)	954		(5		)
Mortgage-backed securities:
Residential	—		—			15		—			15		—
Commercial	42		(1		)	170		(2		)	212		(3		)
Asset-backed securities	755		(5		)	70		(1		)	825		(6		)
Corporate debt securities	70		—			1,486		(24		)	1,556		(24		)
Total debt securities	$	1,177	$	(7	)	$	4,031	$	(54	)	$	5,208	$	(61	)

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	)

Approximately 97% 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 March 31, 2019. 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 12%. 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 690 positions out of a total of approximately 1,430 positions at March 31, 2019. All issuers of securities we own that were trading at an unrealized loss at March 31, 2019 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 March 31, 2019, 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 March 31, 2019.

The detail of realized gains (losses) related to investment securities and included within investment income was as follows for the three months ended March 31, 2019 and 2018:



	Three months ended March 31,
	2019			2018
	(in millions)
Gross realized gains	$	10		$	32
Gross realized losses	(12		)	(3		)
Net realized capital (losses) gains	$	(2	)	$	29

There were no material other-than-temporary impairments for the three months ended March 31, 2019 or 2018.

The contractual maturities of debt securities available for sale at March 31, 2019, 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	$	829	$	828
Due after one year through five years	2,514		2,511
Due after five years through ten years	1,655		1,652
Due after ten years	535		540
Mortgage and asset-backed securities	4,773		4,767
Total debt securities	$	10,306	$	10,298