Securities

A summary of the amortized cost and estimated fair value of securities, excluding trading securities, is presented below.



	September 30, 2015								December 31, 2014
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Held to Maturity
U.S. Treasury	$	249,331	$	10,904	$	—	$	260,235	$	249,009	$	14,604	$	—	$	263,613
Residential mortgage-backed securities	6,723		83		—		6,806		8,012		92		—		8,104
States and political subdivisions	2,420,081		37,594		8,458		2,449,217		2,668,115		34,243		9,035		2,693,323
Other	1,350		—		—		1,350		1,350		—		—		1,350
Total	$	2,677,485	$	48,581	$	8,458	$	2,717,608	$	2,926,486	$	48,939	$	9,035	$	2,966,390
Available for Sale
U.S. Treasury	$	3,712,808	$	69,579	$	—	$	3,782,387	$	3,783,899	$	30,594	$	3,241	$	3,811,252
Residential mortgage-backed securities	1,077,507		53,490		466		1,130,531		1,331,114		68,027		417		1,398,724
States and political subdivisions	3,933,101		93,340		9,108		4,017,333		3,104,563		104,500		156		3,208,907
Other	42,437		—		—		42,437		42,371		—		—		42,371
Total	$	8,765,853	$	216,409	$	9,574	$	8,972,688	$	8,261,947	$	203,121	$	3,814	$	8,461,254

All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At September 30, 2015, approximately 97.7% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 62.8% are either guaranteed by the Texas Permanent School Fund, which has a “triple A” insurer financial strength rating, or secured by U.S. Treasury securities via defeasance of the debt by the issuers. Securities with limited marketability, such as stock in the Federal Reserve Bank and the Federal Home Loan Bank, are carried at cost and are reported as other available for sale securities in the above table. The carrying value of securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $3.3 billion at September 30, 2015 and $3.0 billion and December 31, 2014.

During 2012, we reclassified certain securities from available for sale to held to maturity. The securities had an aggregate fair value of $2.3 billion with an aggregate net unrealized gain of $165.7 million ($107.7 million, net of tax) on the date of the transfer. The net unamortized, unrealized gain on the transferred securities included in accumulated other comprehensive income in the accompanying balance sheet as of September 30, 2015 totaled $69.0 million ($44.8 million, net of tax). This amount will be amortized out of accumulated other comprehensive income over the remaining life of the underlying securities as an adjustment of the yield on those securities.

As of September 30, 2015, securities with unrealized losses, segregated by length of impairment, were as follows:



	Less than 12 Months				More than 12 Months				Total
	Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
Held to Maturity
States and political subdivisions	$	295,618	$	3,233	$	211,796	$	5,225	$	507,414	$	8,458
Total	$	295,618	$	3,233	$	211,796	$	5,225	$	507,414	$	8,458
Available for Sale
Residential mortgage-backed securities	$	25,945	$	148	$	11,326	$	318	$	37,271	$	466
States and political subdivisions	786,267		9,108		—		—		786,267		9,108
Total	$	812,212	$	9,256	$	11,326	$	318	$	823,538	$	9,574

Declines in the fair value of held-to-maturity and available-for-sale securities below their cost that are deemed to be other than temporary are reflected in earnings as realized losses to the extent the impairment is related to credit losses. The amount of the impairment related to other factors is recognized in other comprehensive income. In estimating other-than-temporary impairment losses, management considers, among other things, (i) the length of time and the extent to which the fair value has been less than cost, (ii) the financial condition and near-term prospects of the issuer and (iii) the intent and ability to retain our investment in the issuer for a period of time sufficient to allow for any anticipated recovery in cost.

Management has the ability and intent to hold the securities classified as held to maturity in the table above until they mature, at which time we will receive full value for the securities. Furthermore, as of September 30, 2015, management does not have the intent to sell any of the securities classified as available for sale in the table above and believes that it is more likely than not that we will not have to sell any such securities before a recovery of cost. Any unrealized losses are largely due to increases in market interest rates over the yields available at the time the underlying securities were purchased. The fair value is expected to recover as the bonds approach their maturity date or repricing date or if market yields for such investments decline. Management does not believe any of the securities are impaired due to reasons of credit quality. Accordingly, as of September 30, 2015, management believes the impairments detailed in the table above are temporary and no impairment loss has been realized in our consolidated income statement.

The amortized cost and estimated fair value of securities, excluding trading securities, at September 30, 2015 are presented below by contractual maturity. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Residential mortgage-backed securities and equity securities are shown separately since they are not due at a single maturity date.



	Held to Maturity				Available for Sale
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	156,527	$	159,683	$	279,202	$	281,771
Due after one year through five years	783,310		833,089		3,146,728		3,196,000
Due after five years through ten years	210,771		211,787		1,444,658		1,495,873
Due after ten years	1,520,154		1,506,243		2,775,321		2,826,076
Residential mortgage-backed securities	6,723		6,806		1,077,507		1,130,531
Equity securities	—		—		42,437		42,437
Total	$	2,677,485	$	2,717,608	$	8,765,853	$	8,972,688

Sales of securities available for sale were as follows:



	Three Months Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Proceeds from sales	$	4,497,202		$	3,649,954		$	4,721,189		$	3,651,982
Gross realized gains	—			33			228			36
Gross realized losses	(52		)	—			(52		)	(1		)
Tax (expense) benefit of securities gains/losses	18			(11		)	(62		)	(12		)

Premium amortization and discount accretion included in interest income on securities was as follows:



	Three Months Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Premium amortization	$	(21,354	)	$	(17,471	)	$	(62,192	)	$	(48,862	)
Discount accretion	2,519			1,865			7,846			4,897
Net (premium amortization) discount accretion	$	(18,835	)	$	(15,606	)	$	(54,346	)	$	(43,965	)

Trading account securities, at estimated fair value, were as follows:



	September 30, 2015		December 31, 2014
U.S. Treasury	$	16,244	$	15,339
States and political subdivisions	3,428		87
Total	$	19,672	$	15,426

Net gains and losses on trading account securities were as follows:



	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015			2014
Net gain on sales transactions	$	277	$	167	$	835		$	660
Net mark-to-market gains (losses)	6		13		(40		)	10
Net gain (loss) on trading account securities	$	283	$	180	$	795		$	670