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Securities
 12 Months Ended
Dec. 31, 2025
Investments, Debt and Equity Securities [Abstract]  
Securities Securities
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 
All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. The carrying value of held-to-maturity securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $1.4 billion at both December 31, 2025 and 2024, respectively. Accrued interest receivable on held-to-maturity securities totaled $37.2 million and $37.8 million at December 31, 2025 and 2024, respectively and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets.
The allowance for credit losses on held-to-maturity securities is a contra-asset valuation account that is deducted from the amortized cost basis of held-to-maturity securities to present the net amount expected to be collected. Management measures expected credit losses on held-to-maturity securities on a collective basis by major security type with each type sharing similar risk characteristics and considers historical credit loss information that is adjusted for current conditions and reasonable and supportable forecasts. With regard to U.S. Treasury and residential mortgage-backed securities issued by the U.S. government, or agencies thereof, it is expected that the securities will not be settled at prices less than the amortized cost bases of the securities as such securities are backed by the full faith and credit of and/or guaranteed by the U.S. government. Accordingly, no allowance for credit losses has been recorded for these securities. With regard to securities issued by States and political subdivisions and other held-to-maturity securities, management considers (i) issuer bond ratings, (ii) historical loss rates for given bond ratings, (iii) whether issuers continue to make timely principal and interest payments under the contractual terms of the securities, (iv) internal forecasts and (v) whether or not such securities are guaranteed by the Texas Permanent School Fund (“PSF”) or pre-refunded by the issuers.
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 
Historical loss rates associated with securities having similar grades as those in our portfolio have generally not been significant. Furthermore, as of December 31, 2025, there were no past due principal or interest payments associated with these securities. The PSF is a sovereign wealth fund which serves to provide revenues for funding of public primary and secondary education in the State of Texas. Based upon (i) the PSF's AAA insurer financial strength rating, (ii) the PSF's substantial capitalization and excess guarantee capacity and (iii) a zero historical loss rate, no allowance for credit losses has been recorded for securities guaranteed by the PSF as there is no current expectation of credit losses related to these securities. Pre-refunded securities have been defeased by the issuer and are fully secured by cash and/or U.S. Treasury securities held in escrow for payment to holders when the underlying call dates of the securities are reached. Accordingly, no allowance for credit losses has been recorded for securities that have been defeased as there is no current expectation of credit losses related to these securities.
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 
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 
All mortgage-backed securities included in the above table were issued by U.S. government agencies and corporations. At December 31, 2025, all of the securities in our available for sale municipal bond portfolio were issued by the State of Texas or political subdivisions or agencies within the State of Texas, of which approximately 71.5% are either guaranteed by the PSF or have been pre-refunded. 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 available-for-sale securities pledged to secure public funds, trust deposits, repurchase agreements and for other purposes, as required or permitted by law was $6.6 billion and $6.2 billion at December 31, 2025 and 2024, respectively. Accrued interest receivable on available-for-sale securities totaled $122.5 million and $104.9 million at December 31, 2025 and 2024, respectively, and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets.
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 $1,126,817 
As of December 31, 2025, no allowance for credit losses has been recognized on available for sale securities in an unrealized loss position as management does not believe any of the securities are impaired due to reasons of credit quality. This is based upon our analysis of the underlying risk characteristics, including credit ratings, and other qualitative factors related to our available for sale securities and in consideration of our historical credit loss experience and internal forecasts. The issuers of these securities continue to make timely principal and interest payments under the contractual terms of the securities. Furthermore, management does not have the intent to sell any of the securities classified as available for sale in the table above and believes that it is more likely than not that we will not have to sell any such securities before a recovery of cost. The unrealized losses are due to increases in market interest rates over the yields available at the time the underlying securities were purchased. The fair value is expected to recover as the securities approach their maturity date or repricing date or if market yields for such investments decline.
Contractual Maturities. The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of December 31, 2025. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $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 
Sales of Securities. Sales of securities available for sale were as follows:
202520242023
Proceeds from sales$45,372 $123,254 $1,904,067 
Gross realized gains43 426 5,758 
Gross realized losses(893)(522)(5,692)
Tax benefit (expense) related to securities gains/losses179 20 (14)
Premiums and Discounts. Premium amortization and discount accretion included in interest income on securities was as follows:
202520242023
Premium amortization$(60,337)$(66,257)$(85,506)
Discount accretion20,737 21,044 21,613 
Net (premium amortization) discount accretion$(39,600)$(45,213)$(63,893)
Trading Account Securities. 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 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