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Securities | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Securities | Securities Securities. Year-end securities held to maturity and available for sale consisted of the following:
| 2017 |  | 2016
| 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 | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 249,889 |  |  | $ | 1,762 |  |  | $ | — |  |  | $ | 251,651 |
Residential mortgage-backed securities | 3,610 |  |  | 15 |  |  | 38 |  |  | 3,587 |  |  | 4,511 |  |  | 39 |  |  | — |  |  | 4,550 |
States and political subdivisions | 1,428,488 |  |  | 26,462 |  |  | 2,746 |  |  | 1,452,204 |  |  | 1,994,710 |  |  | 16,821 |  |  | 6,335 |  |  | 2,005,196 |
Other | — |  |  | — |  |  | — |  |  | — |  |  | 1,350 |  |  | — |  |  | — |  |  | 1,350 |
Total | $ | 1,432,098 |  |  | $ | 26,477 |  |  | $ | 2,784 |  |  | $ | 1,455,791 |  |  | $ | 2,250,460 |  |  | $ | 18,622 |  |  | $ | 6,335 |  |  | $ | 2,262,747 |
Available for Sale: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury | $ | 3,453,391 |  |  | $ | 7,494 |  |  | $ | 15,732 |  |  | $ | 3,445,153 |  |  | $ | 4,003,692 |  |  | $ | 24,984 |  |  | $ | 8,945 |  |  | $ | 4,019,731 |
Residential mortgage-backed securities | 648,288 |  |  | 19,048 |  |  | 2,250 |  |  | 665,086 |  |  | 756,072 |  |  | 30,388 |  |  | 1,293 |  |  | 785,167 |
States and political subdivisions | 6,185,711 |  |  | 167,293 |  |  | 16,795 |  |  | 6,336,209 |  |  | 5,403,918 |  |  | 50,101 |  |  | 98,134 |  |  | 5,355,885 |
Other | 42,561 |  |  | — |  |  | — |  |  | 42,561 |  |  | 42,494 |  |  | — |  |  | — |  |  | 42,494 |
Total | $ | 10,329,951 |  |  | $ | 193,835 |  |  | $ | 34,777 |  |  | $ | 10,489,009 |  |  | $ | 10,206,176 |  |  | $ | 105,473 |  |  | $ | 108,372 |  |  | $ | 10,203,277 |
All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At December 31, 2017, approximately 98.2% of the securities in our municipal bond portfolio were issued by political subdivisions or agencies within the State of Texas, of which approximately 67.8% are either guaranteed by the Texas Permanent School Fund, which has a “triple-A” insurer financial strength rating, or are 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 $3.8 billion at December 31, 2017 and $3.9 billion at December 31, 2016. 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 remaining transferred securities included in accumulated other comprehensive income in the accompanying balance sheet totaled $11.6 million ($7.5 million, net of tax) at December 31, 2017 and $27.7 million ($18.0 million, net of tax) at December 31, 2016. 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. 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
2017 |  |  |  |  |  |  |  |  |  |  |
Held to Maturity: |  |  |  |  |  |  |  |  |  |  |
Residential mortgage-backed securities | $ | 2,694 |  |  | $ | 38 |  |  | $ | — |  |  | $ | — |  |  | $ | 2,694 |  |  | $ | 38 |
States and political subdivisions | 28,591 |  |  | 58 |  |  | 74,113 |  |  | 2,688 |  |  | 102,704 |  |  | 2,746 |
Total | $ | 31,285 |  |  | $ | 96 |  |  | $ | 74,113 |  |  | $ | 2,688 |  |  | $ | 105,398 |  |  | $ | 2,784 |
Available for Sale: |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury | $ | 2,336,081 |  |  | $ | 9,861 |  |  | $ | 517,575 |  |  | $ | 5,871 |  |  | $ | 2,853,656 |  |  | $ | 15,732 |
Residential mortgage-backed securities | 144,264 |  |  | 949 |  |  | 45,436 |  |  | 1,301 |  |  | 189,700 |  |  | 2,250 |
States and political subdivisions | 148,575 |  |  | 1,194 |  |  | 838,329 |  |  | 15,601 |  |  | 986,904 |  |  | 16,795 |
Total | $ | 2,628,920 |  |  | $ | 12,004 |  |  | $ | 1,401,340 |  |  | $ | 22,773 |  |  | $ | 4,030,260 |  |  | $ | 34,777 |
| Less than 12 Months |  | More than 12 Months |  | Total
| Estimated Fair Value |  | Unrealized Losses |  | Estimated Fair Value |  | Unrealized Losses |  | Estimated Fair Value |  | Unrealized Losses
2016 |  |  |  |  |  |  |  |  |  |  |
Held to Maturity: |  |  |  |  |  |  |  |  |  |  |
States and political subdivisions | $ | 425,896 |  |  | $ | 2,596 |  |  | $ | 183,245 |  |  | $ | 3,739 |  |  | $ | 609,141 |  |  | $ | 6,335 |
Total | $ | 425,896 |  |  | $ | 2,596 |  |  | $ | 183,245 |  |  | $ | 3,739 |  |  | $ | 609,141 |  |  | $ | 6,335 |
Available for Sale: |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury | $ | 1,421,216 |  |  | $ | 8,945 |  |  | $ | — |  |  | $ | — |  |  | $ | 1,421,216 |  |  | $ | 8,945 |
Residential mortgage-backed securities | 81,442 |  |  | 1,031 |  |  | 6,413 |  |  | 262 |  |  | 87,855 |  |  | 1,293 |
States and political subdivisions | 2,695,997 |  |  | 98,134 |  |  | — |  |  | — |  |  | 2,695,997 |  |  | 98,134 |
Total | $ | 4,198,655 |  |  | $ | 108,110 |  |  | $ | 6,413 |  |  | $ | 262 |  |  | $ | 4,205,068 |  |  | $ | 108,372 |
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 expect to receive full value for the securities. Furthermore, as of December 31, 2017, 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 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 securities 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, 2017, 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 December 31, 2017 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 | $ | 264,587 |  |  | $ | 267,924 |  |  | $ | 57,390 |  |  | $ | 58,057 |
Due after one year through five years | 159,765 |  |  | 163,819 |  |  | 4,063,797 |  |  | 4,056,508 |
Due after five years through ten years | 386,711 |  |  | 391,784 |  |  | 425,148 |  |  | 439,008 |
Due after ten years | 617,425 |  |  | 628,677 |  |  | 5,092,767 |  |  | 5,227,789 |
Residential mortgage-backed securities | 3,610 |  |  | 3,587 |  |  | 648,288 |  |  | 665,086 |
Equity securities | — |  |  | — |  |  | 42,561 |  |  | 42,561 |
Total | $ | 1,432,098 |  |  | $ | 1,455,791 |  |  | $ | 10,329,951 |  |  | $ | 10,489,009 |
Sales of Securities. During 2016, we sold certain securities issued by municipalities that, based upon our internal credit analysis, had experienced significant deterioration in creditworthiness. The risk exposure presented by these municipalities had increased beyond acceptable levels and we determined that it was reasonably possible that all amounts due would not be collected. In the first case, our credit analysis determined that most of the affected municipalities had 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 had been adversely impacted by the sustained low-level of oil prices. Additionally, some of these municipalities had already been downgraded or had been put on credit watch and were subsequently downgraded by various credit rating agencies. In the second case, we sold certain securities related to a municipality that was unrelated to a reliance on the energy industry. This municipality had experienced significant deterioration in creditworthiness as a result of the emergence of significant funding obligations which resulted in credit downgrades. In both cases, 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:
| 2017 |  | 2016 |  | 2015
Proceeds from sales | $ | — |  |  | $ | 136,719 |  |  | $ | — |
Amortized cost | — |  |  | 132,974 |  |  | — |
Gross realized gains | — |  |  | 3,770 |  |  | — |
Gross realized losses | — |  |  | (25 | ) |  | — |
Tax expense related to securities gains/losses | — |  |  | (1,311 | ) |  | — |
Sales of securities available for sale were as follows:
| 2017 |  | 2016 |  | 2015
Proceeds from sales | $ | 11,963,359 |  |  | $ | 14,847,380 |  |  | $ | 12,683,169 |
Gross realized gains | 1 |  |  | 13,289 |  |  | 228 |
Gross realized losses | (4,942 | ) |  | (2,059 | ) |  | (159 | )
Tax benefit (expense) related to securities gains/losses | 1,729 |  |  | (3,931 | ) |  | (24 | )
Premiums and Discounts. Premium amortization and discount accretion included in interest income on securities was as follows:
| 2017 |  | 2016 |  | 2015
Premium amortization | $ | (97,841 | ) |  | $ | (90,782 | ) |  | $ | (84,467 | )
Discount accretion | 7,908 |  |  | 11,077 |  |  | 10,682 |
Net (premium amortization) discount accretion | $ | (89,933 | ) |  | $ | (79,705 | ) |  | $ | (73,785 | )
Trading Account Securities. Year-end trading account securities, at estimated fair value, were as follows:
| 2017 |  | 2016
U.S. Treasury | $ | 19,210 |  |  | $ | 16,594 |
States and political subdivisions | 1,888 |  |  | 109 |
Total | $ | 21,098 |  |  | $ | 16,703 |
Net gains and losses on trading account securities were as follows:
| 2017 |  | 2016 |  | 2015
Net gain on sales transactions | $ | 1,408 |  |  | $ | 1,236 |  |  | $ | 1,109 |
Net mark-to-market gains (losses) | (43 | ) |  | (157 | ) |  | (53 | )
Net gain on trading account securities | $ | 1,365 |  |  | $ | 1,079 |  |  | $ | 1,056 |

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