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Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Year End Securities Held To Maturity And Available For Sale | 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 |
Year End Securities with Unrealized Losses, Segregated by Length of Impairment | 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 |
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity | 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 |
Held-to-maturity Securities [Table Text Block] | 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 | 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 | )
Premium Amortization and Discount Accretion Included in Income on Securities | 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 | )
Year End Trading Account Securities, at Estimated Fair Value | 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 | 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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