Securities

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



	March 31, 2016								December 31, 2015
	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	$	249,550	$	6,895	$	—	$	256,445	$	249,441	$	7,776	$	—	$	257,217
Residential mortgage-backed securities	5,756		71		2		5,825		6,456		63		4		6,515
States and political subdivisions	2,111,363		55,139		5,088		2,161,414		2,405,762		46,003		6,149		2,445,616
Other	1,350		—		—		1,350		1,350		—		13		1,337
Total	$	2,368,019	$	62,105	$	5,090	$	2,425,034	$	2,663,009	$	53,842	$	6,166	$	2,710,685
Available for Sale
U.S. Treasury	$	3,731,803	$	93,031	$	—	$	3,824,834	$	3,980,986	$	22,041	$	8,507	$	3,994,520
Residential mortgage-backed securities	930,927		45,929		216		976,640		1,000,024		42,142		734		1,041,432
States and political subdivisions	4,067,577		156,585		876		4,223,286		3,996,113		133,305		1,459		4,127,959
Other	42,469		—		—		42,469		42,447		—		—		42,447
Total	$	8,772,776	$	295,545	$	1,092	$	9,067,229	$	9,019,570	$	197,488	$	10,700	$	9,206,358

All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At March 31, 2016, 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 68.0% 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 table above. 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 $3.4 billion at March 31, 2016 and $3.9 billion and December 31, 2015.

During the fourth quarter of 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. Some of these securities were sold during the first quarter of 2016, as more fully discussed below. The net unamortized, unrealized gain on the remaining transferred securities included in accumulated other comprehensive income in the accompanying balance sheet as of March 31, 2016 totaled $51.8 million ($33.7 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.

Unrealized Losses. As of March 31, 2016, 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
Residential mortgage-backed securities	$	2,352	$	2	$	—	$	—	$	2,352	$	2
States and political subdivisions	60,055		1,016		142,187		4,072		202,242		5,088
Total	$	62,407	$	1,018	$	142,187	$	4,072	$	204,594	$	5,090
Available for Sale
Residential mortgage-backed securities	$	227	$	2	$	24,321	$	214	$	24,548	$	216
States and political subdivisions	164,280		449		67,609		427		231,889		876
Total	$	164,507	$	451	$	91,930	$	641	$	256,437	$	1,092

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 our 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 March 31, 2016, 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 March 31, 2016, management believes the impairments detailed in the table above are temporary and no impairment loss has been realized in our consolidated income statement.

Contractual Maturities. The amortized cost and estimated fair value of securities, excluding trading securities, at March 31, 2016 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	$	576,622	$	594,737	$	55,687	$	57,014
Due after one year through five years	323,002		343,717		3,504,555		3,572,118
Due after five years through ten years	291,767		297,659		1,235,012		1,293,154
Due after ten years	1,170,872		1,183,096		3,004,126		3,125,834
Residential mortgage-backed securities	5,756		5,825		930,927		976,640
Equity securities	—		—		42,469		42,469
Total	$	2,368,019	$	2,425,034	$	8,772,776	$	9,067,229

Sales of Securities. During the three months ended March 31, 2016, a portion of the securities we sold included certain securities that were issued by municipalities and special-purpose districts under municipal control (together referred to as “municipalities”) within the State of Texas that have been significantly impacted by the significant decline in market oil prices due to the fact that their tax bases are heavily reliant on the energy industry relative to other sectors of the economy. Specifically, the revenues of these municipalities have been adversely impacted by the sustained low-level of oil prices. Additionally, some of these municipalities had been put on credit watch and have subsequently received downgrades by credit rating agencies. In consideration of this, along with our own internal credit analysis, we determined that the creditworthiness of these municipalities had significantly deteriorated and that it was reasonably possible that all amounts due would not be collected. Because this increased risk exposure exceeded acceptable levels, we sold certain securities issued by those municipalities. We did not sell any securities issued by these municipalities that are either guaranteed by the Texas Permanent School Fund or secured by U.S. Treasury securities via defeasance of the debt by the issuers because, as a result of these credit enhancements, the collectibility of these securities is not in doubt. Some of the securities we sold were classified as held to maturity prior to their sale. Despite their classification as held to maturity, we believe the sale of these securities was merited and permissible under the applicable accounting guidelines because of the significant deterioration in the creditworthiness of the issuers.

Sales of securities held to maturity were as follows:



	Three Months Ended March 31,
	2016			2015
Proceeds from sales	$	135,610		$	—
Amortized cost	131,840			—
Gross realized gains	3,770			—
Gross realized losses	—			—
Tax (expense) benefit of securities gains/losses	(1,319		)	—

Sales of securities available for sale were as follows:



	Three Months Ended March 31,
	2016			2015
Proceeds from sales	$	1,060,196		$	223,987
Gross realized gains	11,133			228
Gross realized losses	—			—
Tax (expense) benefit of securities gains/losses	(3,897		)	(80		)

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



	Three Months Ended March 31,
	2016			2015
Premium amortization	$	(22,340	)	$	(20,006	)
Discount accretion	2,615			2,698
Net (premium amortization) discount accretion	$	(19,725	)	$	(17,308	)

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



	March 31, 2016		December 31, 2015
U.S. Treasury	$	16,544	$	16,443
States and political subdivisions	479		136
Total	$	17,023	$	16,579

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



	Three Months Ended March 31,
	2016		2015
Net gain on sales transactions	$	302	$	280
Net mark-to-market gains (losses)	1		(14		)
Net gain (loss) on trading account securities	$	303	$	266