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Securities (Tables)
 9 Months Ended
Sep. 30, 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 September 30, 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
September 30, 2026
Residential mortgage-backed securities
$1,058,075 $— $74,954 $983,121 $— $1,058,075 
States and political subdivisions
2,303,713 153 335,707 1,968,159 (500)2,303,213 
Other1,500 — 2 1,498 — 1,500 
Total$3,363,288 $153 $410,663 $2,952,778 $(500)$3,362,788 
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 September 30, 2026 and December 31, 2025:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
September 30, 2026
Aaa/AAA$303,459 $1,448,555 $6,126 $43,085 $1,801,225 $— 
Aa/AA484,086 — 13,580 — 497,666 — 
A4,822 — — — 4,822 — 
Not rated— — — — — 1,500 
Total$792,367 $1,448,555 $19,706 $43,085 $2,303,713 $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 and nine months ended September 30, 2026 and 2025.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2026202520262025
Beginning balance$500 $310 $500 $310 
Credit loss expense (benefit)— 190 — 190 
Ending balance$500 $500 $500 $500 
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 September 30, 2026 and December 31, 2025, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
September 30, 2026
U.S. Treasury$2,564,377 $— $180,863 $— $2,383,514 
Residential mortgage-backed securities
10,836,499 43 1,295,476 — 9,541,066 
States and political subdivisions
5,662,848 641 531,694 — 5,131,795 
Other42,253 — — — 42,253 
Total$19,105,977 $684 $2,008,033 $— $17,098,628 
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 September 30, 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$538,419 $6,137 $1,845,095 $174,726 $2,383,514 $180,863 
Residential mortgage-backed securities5,856,558 267,168 3,676,684 1,028,308 9,533,242 1,295,476 
States and political subdivisions3,085,479 203,857 1,833,440 327,837 4,918,919 531,694 
Total$9,480,456 $477,162 $7,355,219 $1,530,871 $16,835,675 $2,008,033 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of September 30, 2026. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $493,164 $11,203 $553,708 $1,058,075 
States and political subdivisions32,912 6,453 129,413 2,134,935 2,303,713 
Other1,500 — — — 1,500 
Total$34,412 $499,617 $140,616 $2,688,643 $3,363,288 
Estimated Fair Value
Residential mortgage-backed securities$— $453,672 $9,101 $520,348 $983,121 
States and political subdivisions32,884 6,389 116,964 1,811,922 1,968,159 
Other1,498 — — — 1,498 
Total$34,382 $460,061 $126,065 $2,332,270 $2,952,778 
Available For Sale
Amortized Cost
U.S. Treasury$449,045 $1,938,321 $— $177,011 $2,564,377 
Residential mortgage-backed securities15 10,029 1,766 10,824,689 10,836,499 
States and political subdivisions90,600 304,792 399,285 4,868,171 5,662,848 
Other— — — — 42,253 
Total$539,660 $2,253,142 $401,051 $15,869,871 $19,105,977 
Estimated Fair Value
U.S. Treasury$447,555 $1,818,975 $— $116,984 $2,383,514 
Residential mortgage-backed securities15 10,012 1,744 9,529,295 9,541,066 
States and political subdivisions90,509 293,508 352,005 4,395,773 5,131,795 
Other— — — — 42,253 
Total$538,079 $2,122,495 $353,749 $14,042,052 $17,098,628 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2026202520262025
Proceeds from sales$27,916 $2,651 $27,916 $41,207 
Gross realized gains— — — 43 
Gross realized losses(7,512)— (7,512)(57)
Tax (expense) benefit of securities gains/losses1,578 — 1,578 3 
Investment Income Premium amortization and discount accretion included in interest income on securities were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2026202520262025
Premium amortization$(11,212)$(14,220)$(36,563)$(44,311)
Discount accretion8,070 5,441 30,233 15,404 
Net (premium amortization) discount accretion$(3,142)$(8,779)$(6,330)$(28,907)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
September 30,
2026		December 31,
2025
U.S. Treasury$37,986 $36,650 
States and political subdivisions11,661 954 
Total$49,647 $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
September 30,		Nine Months Ended
September 30,
2026202520262025
Net gain on sales transactions$1,266 $1,323 $3,872 $4,111 
Net mark-to-market gains (losses)(766)28 (778)— 
Net gain (loss) on trading account securities$500 $1,351 $3,094 $4,111