Securities

Year-end securities held to maturity and available for sale consisted of the following:



	2015								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	$	249,441	$	7,776	$	—	$	257,217	$	249,009	$	14,604	$	—	$	263,613
Residential mortgage-backed securities	6,456		63		4		6,515		8,012		92		—		8,104
States and political subdivisions	2,405,762		46,003		6,149		2,445,616		2,668,115		34,243		9,035		2,693,323
Other	1,350		—		13		1,337		1,350		—		—		1,350
Total	$	2,663,009	$	53,842	$	6,166	$	2,710,685	$	2,926,486	$	48,939	$	9,035	$	2,966,390
Available for Sale:
U. S. Treasury	$	3,980,986	$	22,041	$	8,507	$	3,994,520	$	3,783,899	$	30,594	$	3,241	$	3,811,252
Residential mortgage-backed securities	1,000,024		42,142		734		1,041,432		1,331,114		68,027		417		1,398,724
States and political subdivisions	3,996,113		133,305		1,459		4,127,959		3,104,563		104,500		156		3,208,907
Other	42,447		—		—		42,447		42,371		—		—		42,371
Total	$	9,019,570	$	197,488	$	10,700	$	9,206,358	$	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 December 31, 2015, approximately 97.8% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 62.4% 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 $3.9 billion at December 31, 2015 and $3.0 billion at 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 totaled $60.3 million ($39.2 million, net of tax) at December 31, 2015 and $93.9 million ($61.0 million, net of tax) at December 31, 2014. 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.

Year-end 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
2015
Held to Maturity:
Residential mortgage-backed securities	$	900	$	4	$	—	$	—	$	900	$	4
States and political subdivisions	146,854		1,325		202,423		4,824		349,277		6,149
Other	1,337		13		—		—		1,337		13
Total	$	149,091	$	1,342	$	202,423	$	4,824	$	351,514	$	6,166
Available for Sale:
U.S. Treasury	$	886,087	$	8,507	$	—	$	—	$	886,087	$	8,507
Residential mortgage-backed securities	21,392		212		17,781		522		39,173		734
States and political subdivisions	120,782		1,237		18,485		222		139,267		1,459
Total	$	1,028,261	$	9,956	$	36,266	$	744	$	1,064,527	$	10,700
2014
Held to Maturity:
States and political subdivisions	$	68,024	$	144	$	683,251	$	8,891	$	751,275	$	9,035
Total	$	68,024	$	144	$	683,251	$	8,891	$	751,275	$	9,035
Available for Sale:
U.S. Treasury	$	1,019,230	$	3,241	$	—	$	—	$	1,019,230	$	3,241
Residential mortgage-backed securities	8,550		42		16,944		375		25,494		417
States and political subdivisions	65,751		156		—		—		65,751		156
Total	$	1,093,531	$	3,439	$	16,944	$	375	$	1,110,475	$	3,814

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 December 31, 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 December 31, 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 December 31, 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	$	161,934	$	163,320	$	793,662	$	796,452
Due after one year through five years	846,629		889,786		2,961,224		2,976,538
Due after five years through ten years	207,488		209,616		1,391,273		1,418,590
Due after ten years	1,440,502		1,441,448		2,830,940		2,930,899
Residential mortgage-backed securities	6,456		6,515		1,000,024		1,041,432
Equity securities	—		—		42,447		42,447
Total	$	2,663,009	$	2,710,685	$	9,019,570	$	9,206,358

Sales of securities available for sale were as follows:



	2015			2014			2013
Proceeds from sales	$	12,683,169		$	12,151,287		$	10,056,060
Gross realized gains	228			39			1,206
Gross realized losses	(159		)	(1		)	(30		)
Tax (expense) benefit of securities gains/losses	(24		)	(13		)	(412		)

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



	2015			2014			2013
Premium amortization	$	(84,467	)	$	(68,070	)	$	(49,112	)
Discount accretion	10,682			6,802			7,191
Net (premium amortization) discount accretion	$	(73,785	)	$	(61,268	)	$	(41,921	)

Year-end trading account securities, at estimated fair value, were as follows:



	2015		2014
U.S. Treasury	$	16,443	$	15,339
States and political subdivisions	136		87
Total	$	16,579	$	15,426

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



	2015			2014		2013
Net gain on sales transactions	$	1,109		$	829	$	878
Net mark-to-market gains (losses)	(53		)	—		(429		)
Net gain on trading account securities	$	1,056		$	829	$	449