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Securities (Tables)
 6 Months Ended
Jun. 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 June 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
June 30, 2023
Residential mortgage-backed securities
$1,265,818 $— $72,286 $1,193,532 $— $1,265,818 
States and political subdivisions
2,424,169 24,288 99,491 2,348,966 (267)2,423,902 
Other1,500 — 69 1,431 — 1,500 
Total$3,691,487 $24,288 $171,846 $3,543,929 $(267)$3,691,220 
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 June 30, 2023 and December 31, 2022:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
June 30, 2023
Aaa/AAA$301,922 $1,544,702 $13,656 $52,860 $1,913,140 $— 
Aa/AA504,928 — 6,101 — 511,029 — 
Not rated— — — — — 1,500 
Total$806,850 $1,544,702 $19,757 $52,860 $2,424,169 $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 six months ended June 30, 2023 and 2022.
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Beginning balance$262 $158 $158 $158 
Credit loss expense (benefit)5 — 109 — 
Ending balance$267 $158 $267 $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 June 30, 2023 and December 31, 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
June 30, 2023
U.S. Treasury$5,456,656 $— $383,768 $— $5,072,888 
Residential mortgage-backed securities
7,725,403 5,105 937,733 — 6,792,775 
States and political subdivisions
5,636,095 1,098 296,177 — 5,341,016 
Other42,578 — — — 42,578 
Total$18,860,732 $6,203 $1,617,678 $— $17,249,257 
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 June 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$1,487,790 $58,237 $3,585,097 $325,531 $5,072,887 $383,768 
Residential mortgage-backed securities2,176,299 65,646 4,239,828 872,087 6,416,127 937,733 
States and political subdivisions3,391,385 45,640 1,543,520 250,537 4,934,905 296,177 
Total$7,055,474 $169,523 $9,368,445 $1,448,155 $16,423,919 $1,617,678 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of June 30, 2023. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $— $513,530 $752,288 $1,265,818 
States and political subdivisions59,282 14,024 37,558 2,313,305 2,424,169 
Other— 1,500 — — 1,500 
Total$59,282 $15,524 $551,088 $3,065,593 $3,691,487 
Estimated Fair Value
Residential mortgage-backed securities$— $— $450,850 $742,682 $1,193,532 
States and political subdivisions59,242 13,949 36,609 2,239,166 2,348,966 
Other— 1,431 — — 1,431 
Total$59,242 $15,380 $487,459 $2,981,848 $3,543,929 
Available For Sale
Amortized Cost
U. S. Treasury$1,286,197 $2,542,538 $1,435,692 $192,229 $5,456,656 
Residential mortgage-backed securities310 4,985 15,986 7,704,122 7,725,403 
States and political subdivisions439,111 317,371 831,502 4,048,111 5,636,095 
Other— — — — 42,578 
Total$1,725,618 $2,864,894 $2,283,180 $11,944,462 $18,860,732 
Estimated Fair Value
U. S. Treasury$1,257,329 $2,420,131 $1,250,420 $145,008 $5,072,888 
Residential mortgage-backed securities305 4,865 15,898 6,771,707 6,792,775 
States and political subdivisions438,969 315,373 811,631 3,775,043 5,341,016 
Other— — — — 42,578 
Total$1,696,603 $2,740,369 $2,077,949 $10,691,758 $17,249,257 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Proceeds from sales$659,082 $— $1,543,355 $— 
Gross realized gains561 — 5,417 — 
Gross realized losses(528)— (5,363)— 
Tax (expense) benefit of securities gains/losses(7)— (11)— 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Premium amortization$(21,242)$(28,053)$(47,167)$(57,113)
Discount accretion4,843 2,482 10,248 3,888 
Net (premium amortization) discount accretion$(16,399)$(25,571)$(36,919)$(53,225)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
June 30,
2023		December 31,
2022
U.S. Treasury$27,546 $25,879 
States and political subdivisions4,971 2,166 
Total$32,517 $28,045 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Net gain on sales transactions$903 $1,012 $1,871 $1,352 
Net mark-to-market gains (losses)(10)(76)(27)(244)
Net gain (loss) on trading account securities$893 $936 $1,844 $1,108