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Securities (Tables)
 3 Months Ended
Mar. 31, 2026
Investments, Debt and Equity Securities [Abstract]  
Debt Securities, Held-to-maturity [Table Text Block] A summary of the amortized cost, fair value and allowance for credit losses related to securities held to maturity as of March 31, 2026 and December 31, 2025, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
March 31, 2026
Residential mortgage-backed securities
$1,097,711 $251 $31,509 $1,066,453 $— $1,097,711 
States and political subdivisions
2,306,789 4,601 161,874 2,149,516 (500)2,306,289 
Other1,500 — 2 1,498 — 1,500 
Total$3,406,000 $4,852 $193,385 $3,217,467 $(500)$3,405,500 
December 31, 2025
Residential mortgage-backed securities
$1,113,474 $5,952 $29,220 $1,090,206 $— $1,113,474 
States and political subdivisions
2,316,705 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 
Moody's and Standard & Poor's Bond Ratings [Table Text Block]
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, 2026 and December 31, 2025:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
March 31, 2026
Aaa/AAA$300,383 $1,465,645 $6,135 $27,558 $1,799,721 $— 
Aa/AA488,635 — 13,579 — 502,214 — 
A4,854 — — — 4,854 — 
Not rated— — — — — 1,500 
Total$793,872 $1,465,645 $19,714 $27,558 $2,306,789 $1,500 
December 31, 2025
Aaa/AAA$300,477 $1,467,646 $6,140 $35,216 $1,809,479 $— 
Aa/AA488,778 — 13,579 — 502,357 — 
A4,869 — — — 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 [Table Text Block]
The following table details activity in the allowance for credit losses on held-to-maturity securities during the three months ended March 31, 2026 and 2025.
Three Months Ended
March 31,
20262025
Beginning balance$500 $310 
Credit loss expense (benefit)— — 
Ending balance$500 $310 
Amortized Cost, Estimated Fair Value and Allowance for Credit Losses of 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, 2026 and December 31, 2025, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
March 31, 2026
U.S. Treasury$3,444,981 $— $153,427 $— $3,291,554 
Residential mortgage-backed securities
9,021,077 35,936 776,782 — 8,280,231 
States and political subdivisions
5,427,357 15,315 266,352 — 5,176,320 
Other42,501 — — — 42,501 
Total$17,935,916 $51,251 $1,196,561 $— $16,790,606 
December 31, 2025
U.S. Treasury$2,604,852 $22 $148,357 $— $2,456,517 
Residential mortgage-backed securities
8,818,139 54,668 751,013 — 8,121,794 
States and political subdivisions
5,548,304 29,000 227,447 — 5,349,857 
Other42,428 — — — 42,428 
Total$17,013,723 $83,690 $1,126,817 $— $15,970,596 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of March 31, 2026, 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$1,037,165 $235 $2,254,388 $153,192 $3,291,553 $153,427 
Residential mortgage-backed securities1,381,582 12,322 4,121,310 764,460 5,502,892 776,782 
States and political subdivisions1,358,345 16,965 2,328,555 249,387 3,686,900 266,352 
Total$3,777,092 $29,522 $8,704,253 $1,167,039 $12,481,345 $1,196,561 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of March 31, 2026. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $494,165 $11,215 $592,331 $1,097,711 
States and political subdivisions23,885 6,443 104,075 2,172,386 2,306,789 
Other1,500 — — — 1,500 
Total$25,385 $500,608 $115,290 $2,764,717 $3,406,000 
Estimated Fair Value
Residential mortgage-backed securities$— $464,693 $9,668 $592,092 $1,066,453 
States and political subdivisions24,036 6,549 100,927 2,018,004 2,149,516 
Other1,498 — — — 1,498 
Total$25,534 $471,242 $110,595 $2,610,096 $3,217,467 
Available For Sale
Amortized Cost
U. S. Treasury$1,685,183 $1,566,540 $— $193,258 $3,444,981 
Residential mortgage-backed securities40 9,770 2,364 9,008,903 9,021,077 
States and political subdivisions81,488 322,286 550,422 4,473,161 5,427,357 
Other— — — — 42,501 
Total$1,766,711 $1,898,596 $552,786 $13,675,322 $17,935,916 
Estimated Fair Value
U. S. Treasury$1,680,776 $1,471,989 $— $138,789 $3,291,554 
Residential mortgage-backed securities39 9,748 2,408 8,268,036 8,280,231 
States and political subdivisions81,585 320,948 519,651 4,254,136 5,176,320 
Other— — — — 42,501 
Total$1,762,400 $1,802,685 $522,059 $12,660,961 $16,790,606 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
March 31,
20262025
Proceeds from sales$— $38,556 
Gross realized gains— 43 
Gross realized losses— (57)
Tax (expense) benefit of securities gains/losses— 3 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
March 31,
20262025
Premium amortization$(13,534)$(15,013)
Discount accretion9,768 5,068 
Net (premium amortization) discount accretion$(3,766)$(9,945)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
March 31,
2026		December 31,
2025
U.S. Treasury$37,366 $36,650 
States and political subdivisions279 954 
Total$37,645 $37,604 
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:
Three Months Ended
March 31,
20262025
Net gain on sales transactions$1,351 $1,273 
Net mark-to-market gains (losses)(13)(33)
Net gain (loss) on trading account securities$1,338 $1,240