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Securities (Tables)
 9 Months Ended
Sep. 30, 2024
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, 2024 and December 31, 2023, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
September 30, 2024
Residential mortgage-backed securities
$1,211,798 $7,380 $39,632 $1,179,546 $— $1,211,798 
States and political subdivisions
2,356,730 31,823 71,318 2,317,235 (310)2,356,420 
Other1,500 — 10 1,490 — 1,500 
Total$3,570,028 $39,203 $110,960 $3,498,271 $(310)$3,569,718 
December 31, 2023
Residential mortgage-backed securities
$1,250,431 $76 $54,175 $1,196,332 $— $1,250,431 
States and political subdivisions
2,367,807 42,990 76,540 2,334,257 (310)2,367,497 
Other1,500 — 45 1,455 — 1,500 
Total$3,619,738 $43,066 $130,760 $3,532,044 $(310)$3,619,428 
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, 2024 and December 31, 2023:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
September 30, 2024
Aaa/AAA$301,414 $1,522,654 $13,643 $14,528 $1,852,239 $— 
Aa/AA498,379 — 6,112 — 504,491 — 
Not rated— — — — — 1,500 
Total$799,793 $1,522,654 $19,755 $14,528 $2,356,730 $1,500 
December 31, 2023
Aaa/AAA$301,721 $1,541,913 $13,651 $1,401 $1,858,686 $— 
Aa/AA
503,016 — 6,105 — 509,121 — 
Not rated— — — — — 1,500 
Total$804,737 $1,541,913 $19,756 $1,401 $2,367,807 $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, 2024 and 2023.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2024202320242023
Beginning balance$310 $267 $310 $158 
Credit loss expense (benefit)— 43 — 152 
Ending balance$310 $310 $310 $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 September 30, 2024 and December 31, 2023, is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
September 30, 2024
U.S. Treasury$4,188,623 $— $200,877 $— $3,987,746 
Residential mortgage-backed securities
7,418,797 22,023 748,157 — 6,692,663 
States and political subdivisions
4,799,347 9,228 212,015 — 4,596,560 
Other43,074 — — — 43,074 
Total$16,449,841 $31,251 $1,161,049 $— $15,320,043 
December 31, 2023
U.S. Treasury$5,212,894 $— $285,305 $— $4,927,589 
Residential mortgage-backed securities
7,463,954 9,066 876,338 — 6,596,682 
States and political subdivisions
5,245,721 5,762 240,152 — 5,011,331 
Other42,769 — — — 42,769 
Total$17,965,338 $14,828 $1,401,795 $— $16,578,371 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of September 30, 2024, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position.
Less than 12 MonthsMore than 12 MonthsTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
U.S. Treasury$— $— $3,987,746 $200,877 $3,987,746 $200,877 
Residential mortgage-backed securities719 2 5,093,851 748,155 5,094,570 748,157 
States and political subdivisions325,850 919 3,633,545 211,096 3,959,395 212,015 
Total$326,569 $921 $12,715,142 $1,160,128 $13,041,711 $1,161,049 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of September 30, 2024. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $391,052 $131,149 $689,597 $1,211,798 
States and political subdivisions7,608 17,701 51,795 2,279,626 2,356,730 
Other1,500 — — — 1,500 
Total$9,108 $408,753 $182,944 $2,969,223 $3,570,028 
Estimated Fair Value
Residential mortgage-backed securities$— $362,510 $120,099 $696,937 $1,179,546 
States and political subdivisions7,599 17,916 50,384 2,241,336 2,317,235 
Other1,490 — — — 1,490 
Total$9,089 $380,426 $170,483 $2,938,273 $3,498,271 
Available For Sale
Amortized Cost
U. S. Treasury$1,173,908 $2,623,994 $198,031 $192,690 $4,188,623 
Residential mortgage-backed securities232 999 12,927 7,404,639 7,418,797 
States and political subdivisions114,631 240,915 818,886 3,624,915 4,799,347 
Other— — — — 43,074 
Total$1,288,771 $2,865,908 $1,029,844 $11,222,244 $16,449,841 
Estimated Fair Value
U. S. Treasury$1,166,473 $2,502,945 $171,836 $146,492 $3,987,746 
Residential mortgage-backed securities231 1,015 13,011 6,678,406 6,692,663 
States and political subdivisions114,501 240,215 794,936 3,446,908 4,596,560 
Other— — — — 43,074 
Total$1,281,205 $2,744,175 $979,783 $10,271,806 $15,320,043 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2024202320242023
Proceeds from sales$60,591 $360,712 $60,591 $1,904,067 
Gross realized gains538 341 538 5,758 
Gross realized losses(522)(329)(522)(5,692)
Tax (expense) benefit of securities gains/losses(3)(3)(3)(14)
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2024202320242023
Premium amortization$(16,078)$(19,367)$(50,601)$(66,534)
Discount accretion5,287 5,672 15,024 15,920 
Net (premium amortization) discount accretion$(10,791)$(13,695)$(35,577)$(50,614)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
September 30,
2024		December 31,
2023
U.S. Treasury$32,261 $30,265 
States and political subdivisions92 1,452 
Total$32,353 $31,717 
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,
2024202320242023
Net gain on sales transactions$1,250 $1,093 $3,632 $2,964 
Net mark-to-market gains (losses)16 (25)(85)(52)
Net gain (loss) on trading account securities$1,266 $1,068 $3,547 $2,912