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Securities (Tables)
 12 Months Ended
Dec. 31, 2024
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, 2024 and 2023 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
December 31, 2024
Residential mortgage-backed securities$1,193,840 $— $71,076 $1,122,764 $— $1,193,840 
States and political subdivisions2,338,745 13,954 116,414 2,236,285 (310)2,338,435 
Other1,500 — 3 1,497 — 1,500 
Total$3,534,085 $13,954 $187,493 $3,360,546 $(310)$3,533,775 
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 
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, 2024:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
Aaa/AAA$301,310 $1,504,951 $13,640 $14,531 $1,834,432 $— 
Aa/AA
498,198 — 6,115 — 504,313 — 
Not rated— — — — — 1,500 
Total$799,508 $1,504,951 $19,755 $14,531 $2,338,745 $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.
202420232022
Beginning balance$310 $158 $158 
Credit loss expense (benefit)— 152 — 
Ending balance$310 $310 $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, 2024 and 2023 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
December 31, 2024
U.S. Treasury$3,692,215 $— $249,895 $— $3,442,320 
Residential mortgage-backed securities8,024,704 2,352 1,029,154 — 6,997,902 
States and political subdivisions4,842,060 2,493 284,329 — 4,560,224 
Other43,179 — — — 43,179 
Total$16,602,158 $4,845 $1,563,378 $— $15,043,625 
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 
Year End Securities with Unrealized Losses, Segregated by Length of Impairment
The table below summarizes, as of December 31, 2024, 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
$— $— $3,442,320 $249,895 $3,442,320 $249,895 
Residential mortgage-backed securities1,289,852 26,420 4,725,498 1,002,734 6,015,350 1,029,154 
States and political subdivisions
836,599 6,961 3,490,644 277,368 4,327,243 284,329 
Total$2,126,451 $33,381 $11,658,462 $1,529,997 $13,784,913 $1,563,378 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of December 31, 2024. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $510,490 $11,244 $672,106 $1,193,840 
States and political subdivisions7,572 17,702 51,690 2,261,781 2,338,745 
Other1,500 — — — 1,500 
Total$9,072 $528,192 $62,934 $2,933,887 $3,534,085 
Estimated Fair Value
Residential mortgage-backed securities$— $459,859 $9,164 $653,741 $1,122,764 
States and political subdivisions7,570 17,695 49,489 2,161,531 2,236,285 
Other1,497 — — — 1,497 
Total$9,067 $477,554 $58,653 $2,815,272 $3,360,546 
Available For Sale
Amortized Cost
U. S. Treasury$1,096,734 $2,204,591 $198,105 $192,785 $3,692,215 
Residential mortgage-backed securities39 869 12,553 8,011,243 8,024,704 
States and political subdivisions284,652 225,414 804,372 3,527,622 4,842,060 
Other— — — — 43,179 
Total$1,381,425 $2,430,874 $1,015,030 $11,731,650 $16,602,158 
Estimated Fair Value
U. S. Treasury$1,088,890 $2,054,258 $165,031 $134,141 $3,442,320 
Residential mortgage-backed securities39 881 12,588 6,984,394 6,997,902 
States and political subdivisions284,459 222,960 765,004 3,287,801 4,560,224 
Other— — — — 43,179 
Total$1,373,388 $2,278,099 $942,623 $10,406,336 $15,043,625 
Schedule of Realized Gain (Loss) Sales of securities available for sale were as follows:
202420232022
Proceeds from sales$123,254 $1,904,067 $— 
Gross realized gains426 5,758 — 
Gross realized losses(522)(5,692)— 
Tax benefit (expense) related to securities gains/losses20 (14)— 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
202420232022
Premium amortization$(66,257)$(85,506)$(110,997)
Discount accretion21,044 21,613 13,597 
Net (premium amortization) discount accretion$(45,213)$(63,893)$(97,400)
Year End Trading Account Securities, at Estimated Fair Value Year-end trading account securities, at estimated fair value, were as follows:
20242023
U.S. Treasury$33,910 $30,265 
States and political subdivisions— 1,452 
Total$33,910 $31,717 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities included in other non-interest income were as follows:
202420232022
Net gain on sales transactions$4,876 $3,843 $3,129 
Net mark-to-market gains (losses)(109)(33)(230)
Net gain on trading account securities$4,767 $3,810 $2,899