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Securities (Tables)
 12 Months Ended
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]  
Debt Securities, Held-to-maturity A summary of the amortized cost, fair value and allowance for credit losses related to securities held to maturity as of December 31, 2023 and 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
December 31, 2023
Residential mortgage-backed securities$1,250,431 $76 $54,175 $1,196,332 $— $1,250,431 
States and political subdivisions2,367,807 42,990 76,540 2,334,257 (310)2,367,497 
Other1,500 — 45 1,455 — 1,500 
Total$3,619,738 $43,066 $130,760 $3,532,044 $(310)$3,619,428 
December 31, 2022
Residential mortgage-backed securities$526,122 $— $65,322 $460,800 $— $526,122 
States and political subdivisions2,111,619 13,048 119,033 2,005,634 (158)2,111,461 
Other1,500 — 69 1,431 — 1,500 
Total$2,639,241 $13,048 $184,424 $2,467,865 $(158)$2,639,083 
Moody's and Standard & Poor's Bond Ratings
The following table summarizes Moody's and/or Standard & Poor's bond ratings for our portfolio of held-to-maturity securities issued by States and political subdivisions and other securities as of December 31, 2023:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
Aaa/AAA$301,721 $1,541,913 $13,651 $1,401 $1,858,686 $— 
Aa/AA
503,016 — 6,105 — 509,121 — 
Not rated— — — — — 1,500 
Total$804,737 $1,541,913 $19,756 $1,401 $2,367,807 $1,500 
Debt Securities, Held-to-maturity, Allowance for Credit Loss
The following table details activity in the allowance for credit losses on held-to-maturity securities.
202320222021
Beginning balance$158 $158 $160 
Credit loss expense (benefit)152 — (2)
Ending balance$310 $158 $158 
Debt Securities, Available-for-sale A summary of the amortized cost, fair value and allowance for credit losses related to securities available for sale as of December 31, 2023 and 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
December 31, 2023
U.S. Treasury$5,212,894 $— $285,305 $— $4,927,589 
Residential mortgage-backed securities7,463,954 9,066 876,338 — 6,596,682 
States and political subdivisions5,245,721 5,762 240,152 — 5,011,331 
Other42,769 — — — 42,769 
Total$17,965,338 $14,828 $1,401,795 $— $16,578,371 
December 31, 2022
U.S. Treasury$5,450,546 $— $398,959 $— $5,051,587 
Residential mortgage-backed securities7,316,824 8,050 948,638 — 6,376,236 
States and political subdivisions7,098,635 9,108 334,388 — 6,773,355 
Other42,427 — — — 42,427 
Total$19,908,432 $17,158 $1,681,985 $— $18,243,605 
Year End Securities with Unrealized Losses, Segregated by Length of Impairment
The table below summarizes, as of December 31, 2023, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position.
Less than 12 MonthsMore than 12 MonthsTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
U.S. Treasury
$— $— $4,927,590 $285,305 $4,927,590 $285,305 
Residential mortgage-backed securities879,380 6,883 5,062,456 869,455 5,941,836 876,338 
States and political subdivisions
1,810,373 10,035 2,652,951 230,117 4,463,324 240,152 
Total$2,689,753 $16,918 $12,642,997 $1,384,877 $15,332,750 $1,401,795 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of December 31, 2023. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $— $512,995 $737,436 $1,250,431 
States and political subdivisions8,170 12,008 52,106 2,295,523 2,367,807 
Other— 1,500 — — 1,500 
Total$8,170 $13,508 $565,101 $3,032,959 $3,619,738 
Estimated Fair Value
Residential mortgage-backed securities$— $— $461,253 $735,079 $1,196,332 
States and political subdivisions8,175 11,999 50,728 2,263,355 2,334,257 
Other— 1,455 — — 1,455 
Total$8,175 $13,454 $511,981 $2,998,434 $3,532,044 
Available For Sale
Amortized Cost
U. S. Treasury$1,534,307 $3,288,365 $197,809 $192,413 $5,212,894 
Residential mortgage-backed securities1,740 1,827 13,985 7,446,402 7,463,954 
States and political subdivisions370,259 183,228 880,659 3,811,575 5,245,721 
Other— — — — 42,769 
Total$1,906,306 $3,473,420 $1,092,453 $11,450,390 $17,965,338 
Estimated Fair Value
U. S. Treasury$1,514,227 $3,101,776 $167,219 $144,367 $4,927,589 
Residential mortgage-backed securities1,718 1,826 13,963 6,579,175 6,596,682 
States and political subdivisions370,111 182,786 859,675 3,598,759 5,011,331 
Other— — — — 42,769 
Total$1,886,056 $3,286,388 $1,040,857 $10,322,301 $16,578,371 
Schedule of Realized Gain (Loss) Sales of securities available for sale were as follows:
202320222021
Proceeds from sales$1,904,067 $— $1,999,891 
Gross realized gains5,758 — 69 
Gross realized losses(5,692)— — 
Tax benefit (expense) related to securities gains/losses(14)— (14)
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
202320222021
Premium amortization$(85,506)$(110,997)$(121,994)
Discount accretion21,613 13,597 2,752 
Net (premium amortization) discount accretion$(63,893)$(97,400)$(119,242)
Year End Trading Account Securities, at Estimated Fair Value Year-end trading account securities, at estimated fair value, were as follows:
20232022
U.S. Treasury$30,265 $25,879 
States and political subdivisions1,452 2,166 
Total$31,717 $28,045 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
202320222021
Net gain on sales transactions$3,843 $3,129 $1,014 
Net mark-to-market gains (losses)(33)(230)(75)
Net gain on trading account securities$3,810 $2,899 $939