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Securities (Tables)
 9 Months Ended
Sep. 30, 2023
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, 2023 and December 31, 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
September 30, 2023
Residential mortgage-backed securities
$1,258,155 $— $113,142 $1,145,013 $— $1,258,155 
States and political subdivisions
2,368,778 48 283,585 2,085,241 (310)2,368,468 
Other1,500 — 63 1,437 — 1,500 
Total$3,628,433 $48 $396,790 $3,231,691 $(310)$3,628,123 
December 31, 2022
Residential mortgage-backed securities
$526,122 $— $65,322 $460,800 $— $526,122 
States and political subdivisions
2,111,619 13,048 119,033 2,005,634 (158)2,111,461 
Other1,500 — 69 1,431 — 1,500 
Total$2,639,241 $13,048 $184,424 $2,467,865 $(158)$2,639,083 
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, 2023 and December 31, 2022:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
September 30, 2023
Aaa/AAA$301,822 $1,542,919 $13,653 $1,091 $1,859,485 $— 
Aa/AA503,190 — 6,103 — 509,293 — 
Not rated— — — — — 1,500 
Total$805,012 $1,542,919 $19,756 $1,091 $2,368,778 $1,500 
December 31, 2022
Aaa/AAA$273,201 $1,422,442 $— $121,961 $1,817,604 $— 
Aa/AA
294,015 — — — 294,015 — 
Not rated— — — — — 1,500 
Total$567,216 $1,422,442 $— $121,961 $2,111,619 $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, 2023 and 2022.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2023202220232022
Beginning balance$267 $158 $158 $158 
Credit loss expense (benefit)43 — 152 — 
Ending balance$310 $158 $310 $158 
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, 2023 and December 31, 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
September 30, 2023
U.S. Treasury$5,459,758 $— $416,606 $— $5,043,152 
Residential mortgage-backed securities
7,589,404 140 1,282,910 — 6,306,634 
States and political subdivisions
5,237,185 — 512,552 — 4,724,633 
Other42,670 — — — 42,670 
Total$18,329,017 $140 $2,212,068 $— $16,117,089 
December 31, 2022
U.S. Treasury$5,450,546 $— $398,959 $— $5,051,587 
Residential mortgage-backed securities
7,316,824 8,050 948,638 — 6,376,236 
States and political subdivisions
7,098,635 9,108 334,388 — 6,773,355 
Other42,427 — — — 42,427 
Total$19,908,432 $17,158 $1,681,985 $— $18,243,605 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of September 30, 2023, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position.
Less than 12 MonthsMore than 12 MonthsTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
U.S. Treasury$— $— $5,043,152 $416,606 $5,043,152 $416,606 
Residential mortgage-backed securities1,486,570 67,981 4,806,198 1,214,929 6,292,768 1,282,910 
States and political subdivisions2,247,673 79,479 2,476,343 433,073 4,724,016 512,552 
Total$3,734,243 $147,460 $12,325,693 $2,064,608 $16,059,936 $2,212,068 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of September 30, 2023. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $— $513,264 $744,891 $1,258,155 
States and political subdivisions7,867 12,052 52,210 2,296,649 2,368,778 
Other— 1,500 — — 1,500 
Total$7,867 $13,552 $565,474 $3,041,540 $3,628,433 
Estimated Fair Value
Residential mortgage-backed securities$— $— $439,829 $705,184 $1,145,013 
States and political subdivisions7,863 11,880 47,029 2,018,469 2,085,241 
Other— 1,437 — — 1,437 
Total$7,863 $13,317 $486,858 $2,723,653 $3,231,691 
Available For Sale
Amortized Cost
U. S. Treasury$1,285,709 $2,794,385 $1,187,343 $192,321 $5,459,758 
Residential mortgage-backed securities772 3,593 15,131 7,569,908 7,589,404 
States and political subdivisions249,063 188,612 861,578 3,937,932 5,237,185 
Other— — — — 42,670 
Total$1,535,544 $2,986,590 $2,064,052 $11,700,161 $18,329,017 
Estimated Fair Value
U. S. Treasury$1,269,227 $2,633,956 $1,011,055 $128,914 $5,043,152 
Residential mortgage-backed securities739 3,486 14,979 6,287,430 6,306,634 
States and political subdivisions247,660 184,085 802,922 3,489,966 4,724,633 
Other— — — — 42,670 
Total$1,517,626 $2,821,527 $1,828,956 $9,906,310 $16,117,089 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2023202220232022
Proceeds from sales$360,712 $— $1,904,067 $— 
Gross realized gains341 — 5,758 — 
Gross realized losses(329)— (5,692)— 
Tax (expense) benefit of securities gains/losses(3)— (14)— 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2023202220232022
Premium amortization$(19,367)$(27,165)$(66,534)$(84,278)
Discount accretion5,672 4,062 15,920 7,950 
Net (premium amortization) discount accretion$(13,695)$(23,103)$(50,614)$(76,328)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
September 30,
2023		December 31,
2022
U.S. Treasury$29,065 $25,879 
States and political subdivisions276 2,166 
Total$29,341 $28,045 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2023202220232022
Net gain on sales transactions$1,093 $1,110 $2,964 $2,462 
Net mark-to-market gains (losses)(25)9 (52)(235)
Net gain (loss) on trading account securities$1,068 $1,119 $2,912 $2,227