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Securities (Tables)
 12 Months Ended
Dec. 31, 2025
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, 2025 and 2024 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
December 31, 2025
Residential mortgage-backed securities$1,113,474 $5,952 $29,220 $1,090,206 $— $1,113,474 
States and political subdivisions2,316,705 10,089 123,068 2,203,726 (500)2,316,205 
Other1,500 — 1 1,499 — 1,500 
Total$3,431,679 $16,041 $152,289 $3,295,431 $(500)$3,431,179 
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 
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, 2025:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
Aaa/AAA$300,477 $1,467,646 $6,140 $35,216 $1,809,479 $— 
Aa/AA
488,778 — 13,579 — 502,357 — 
A
4,869 — — — 4,869 — 
Not rated— — — — — 1,500 
Total$794,124 $1,467,646 $19,719 $35,216 $2,316,705 $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.
202520242023
Beginning balance$310 $310 $158 
Credit loss expense (benefit)190 — 152 
Ending balance$500 $310 $310 
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, 2025 and 2024 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
December 31, 2025
U.S. Treasury$2,604,852 $22 $148,357 $— $2,456,517 
Residential mortgage-backed securities8,818,139 54,668 751,013 — 8,121,794 
States and political subdivisions5,548,304 29,000 227,447 — 5,349,857 
Other42,428 — — — 42,428 
Total$17,013,723 $83,690 $1,126,817 $— $15,970,596 
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 
Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value
The table below summarizes, as of December 31, 2025, 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
$— $— $2,407,177 $148,357 $2,407,177 $148,357 
Residential mortgage-backed securities65,522 192 4,540,847 750,821 4,606,369 751,013 
States and political subdivisions
798,513 9,195 2,758,967 218,252 3,557,480 227,447 
Total$864,035 $9,387 $9,706,991 $1,117,430 $10,571,026 $1,126,817 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of December 31, 2025. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $495,067 $11,221 $607,186 $1,113,474 
States and political subdivisions10,585 28,634 78,947 2,198,539 2,316,705 
Other— 1,500 — — 1,500 
Total$10,585 $525,201 $90,168 $2,805,725 $3,431,679 
Estimated Fair Value
Residential mortgage-backed securities$— $467,368 $9,734 $613,104 $1,090,206 
States and political subdivisions10,597 28,951 78,444 2,085,734 2,203,726 
Other— 1,499 — — 1,499 
Total$10,597 $497,818 $88,178 $2,698,838 $3,295,431 
Available For Sale
Amortized Cost
U. S. Treasury$845,957 $1,367,327 $198,405 $193,163 $2,604,852 
Residential mortgage-backed securities64 9,847 2,424 8,805,804 8,818,139 
States and political subdivisions240,643 340,268 661,716 4,305,677 5,548,304 
Other— — — — 42,428 
Total$1,086,664 $1,717,442 $862,545 $13,304,644 $17,013,723 
Estimated Fair Value
U. S. Treasury$840,252 $1,299,741 $176,274 $140,250 $2,456,517 
Residential mortgage-backed securities63 9,838 2,485 8,109,408 8,121,794 
States and political subdivisions240,683 340,030 636,221 4,132,923 5,349,857 
Other— — — — 42,428 
Total$1,080,998 $1,649,609 $814,980 $12,382,581 $15,970,596 
Schedule of Realized Gain (Loss) Sales of securities available for sale were as follows:
202520242023
Proceeds from sales$45,372 $123,254 $1,904,067 
Gross realized gains43 426 5,758 
Gross realized losses(893)(522)(5,692)
Tax benefit (expense) related to securities gains/losses179 20 (14)
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
202520242023
Premium amortization$(60,337)$(66,257)$(85,506)
Discount accretion20,737 21,044 21,613 
Net (premium amortization) discount accretion$(39,600)$(45,213)$(63,893)
Year End Trading Account Securities, at Estimated Fair Value Year-end trading account securities, at estimated fair value, were as follows:
20252024
U.S. Treasury$36,650 $33,910 
States and political subdivisions954 — 
Total$37,604 $33,910 
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:
202520242023
Net gain on sales transactions$5,549 $4,876 $3,843 
Net mark-to-market gains (losses)9 (109)(33)
Net gain on trading account securities$5,558 $4,767 $3,810