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Securities
 3 Months Ended
Mar. 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 March 31, 2025 and December 31, 2024, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
March 31, 2025
Residential mortgage-backed securities
$1,180,113 $— $51,161 $1,128,952 $— $1,180,113 
States and political subdivisions
2,329,853 3,668 166,723 2,166,798 (310)2,329,543 
Other1,500 — — 1,500 — 1,500 
Total$3,511,466 $3,668 $217,884 $3,297,250 $(310)$3,511,156 
December 31, 2024
Residential mortgage-backed securities
$1,193,840 $— $71,076 $1,122,764 $— $1,193,840 
States and political subdivisions
2,338,745 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 totaled $1.3 billion at March 31, 2025 and $1.4 billion December 31, 2024. Accrued interest receivable on held-to-maturity securities totaled $19.9 million at March 31, 2025 and $37.8 million at December 31, 2024, and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets.
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 March 31, 2025 and December 31, 2024:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
March 31, 2025
Aaa/AAA$300,754 $1,496,775 $13,637 $18,456 $1,829,622 $— 
Aa/AA494,114 — 6,117 — 500,231 — 
Not rated— — — — — 1,500 
Total$794,868 $1,496,775 $19,754 $18,456 $2,329,853 $1,500 
December 31, 2024
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 
The following table details activity in the allowance for credit losses on held-to-maturity securities during the three months ended March 31, 2025 and 2024.
Three Months Ended
March 31,
20252024
Beginning balance$310 $310 
Credit loss expense (benefit)— — 
Ending balance$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 March 31, 2025 and December 31, 2024, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
March 31, 2025
U.S. Treasury$3,695,654 $— $204,459 $— $3,491,195 
Residential mortgage-backed securities
9,577,323 25,377 911,753 — 8,690,947 
States and political subdivisions
4,951,109 911 312,963 — 4,639,057 
Other43,273 — — — 43,273 
Total$18,267,359 $26,288 $1,429,175 $— $16,864,472 
December 31, 2024
U.S. Treasury$3,692,215 $— $249,895 $— $3,442,320 
Residential mortgage-backed securities
8,024,704 2,352 1,029,154 — 6,997,902 
States and political subdivisions
4,842,060 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 March 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 71.3% are either guaranteed by the Texas Permanent School Fund (“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 totaled $6.1 billion at March 31, 2025 and $6.2 billion at December 31, 2024. Accrued interest receivable on available-for-sale securities totaled $86.2 million at March 31, 2025 and $104.9 million at December 31, 2024, respectively, and is included in accrued interest receivable and other assets in the accompanying consolidated balance sheets.
The table below summarizes, as of March 31, 2025, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position.
Less than 12 MonthsMore than 12 MonthsTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
U.S. Treasury$— $— $3,491,195 $204,459 $3,491,195 $204,459 
Residential mortgage-backed securities1,064,145 11,498 4,737,614 900,255 5,801,759 911,753 
States and political subdivisions1,088,059 21,680 3,320,672 291,283 4,408,731 312,963 
Total$2,152,204 $33,178 $11,549,481 $1,395,997 $13,701,685 $1,429,175 
As of March 31, 2025, no allowance for credit losses has been recognized on available for sale securities in an unrealized loss position as management does not believe any of the securities are impaired due to reasons of credit quality. This is based upon our analysis of the underlying risk characteristics, including credit ratings, and other qualitative factors related to our available for sale securities and in consideration of our historical credit loss experience and internal forecasts. The issuers of these securities continue to make timely principal and interest payments under the contractual terms of the securities. Furthermore, management does not have the intent to sell any of the securities classified as available for sale in the table above and believes that it is more likely than not that we will not have to sell any such securities before a recovery of cost. The unrealized losses are due to increases in market interest rates over the yields available at the time the underlying securities were purchased. The fair value is expected to recover as the securities approach their maturity date or repricing date or if market yields for such investments decline.
Contractual Maturities. The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of March 31, 2025. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $509,970 $11,238 $658,905 $1,180,113 
States and political subdivisions5,172 17,506 69,686 2,237,489 2,329,853 
Other— 1,500 — — 1,500 
Total$5,172 $528,976 $80,924 $2,896,394 $3,511,466 
Estimated Fair Value
Residential mortgage-backed securities$— $469,308 $9,451 $650,193 $1,128,952 
States and political subdivisions5,175 17,641 66,544 2,077,438 2,166,798 
Other— 1,500 — — 1,500 
Total$5,175 $488,449 $75,995 $2,727,631 $3,297,250 
Available For Sale
Amortized Cost
U. S. Treasury$1,247,261 $2,057,337 $198,178 $192,878 $3,695,654 
Residential mortgage-backed securities— 745 12,116 9,564,462 9,577,323 
States and political subdivisions123,604 284,478 774,230 3,768,797 4,951,109 
Other— — — — 43,273 
Total$1,370,865 $2,342,560 $984,524 $13,526,137 $18,267,359 
Estimated Fair Value
U. S. Treasury$1,241,789 $1,939,695 $169,938 $139,773 $3,491,195 
Residential mortgage-backed securities— 756 12,162 8,678,029 8,690,947 
States and political subdivisions123,596 280,265 733,505 3,501,691 4,639,057 
Other— — — — 43,273 
Total$1,365,385 $2,220,716 $915,605 $12,319,493 $16,864,472 
Sales of Securities. Sales of available for sale securities were as follows:
Three Months Ended
March 31,
20252024
Proceeds from sales$38,556 $— 
Gross realized gains43 — 
Gross realized losses(57)— 
Tax (expense) benefit of securities gains/losses3 — 
Premiums and Discounts. Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
March 31,
20252024
Premium amortization$(15,013)$(17,953)
Discount accretion5,068 4,705 
Net (premium amortization) discount accretion$(9,945)$(13,248)
Trading Account Securities. Trading account securities, at estimated fair value, were as follows:
March 31,
2025		December 31,
2024
U.S. Treasury$34,452 $33,910 
States and political subdivisions210 — 
Total$34,662 $33,910 
Net gains and losses on trading account securities included in other non-interest income were as follows:
Three Months Ended
March 31,
20252024
Net gain on sales transactions$1,273 $1,159 
Net mark-to-market gains (losses)(33)(19)
Net gain (loss) on trading account securities$1,240 $1,140