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Securities (Tables)
 6 Months Ended
Jun. 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 June 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
June 30, 2024
Residential mortgage-backed securities
$1,225,779 $— $74,853 $1,150,926 $— $1,225,779 
States and political subdivisions
2,359,147 17,319 111,929 2,264,537 (310)2,358,837 
Other1,500 — 26 1,474 — 1,500 
Total$3,586,426 $17,319 $186,808 $3,416,937 $(310)$3,586,116 
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 June 30, 2024 and December 31, 2023:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
June 30, 2024
Aaa/AAA$301,518 $1,538,227 $13,645 $1,090 $1,854,480 $— 
Aa/AA498,557 — 6,110 — 504,667 — 
Not rated— — — — — 1,500 
Total$800,075 $1,538,227 $19,755 $1,090 $2,359,147 $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 six months ended June 30, 2024 and 2023.
Three Months Ended
June 30,		Six Months Ended
June 30,
2024202320242023
Beginning balance$310 $262 $310 $158 
Credit loss expense (benefit)— 5 — 109 
Ending balance$310 $267 $310 $267 
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, 2024 and December 31, 2023 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
June 30, 2024
U.S. Treasury$4,184,938 $— $302,607 $— $3,882,331 
Residential mortgage-backed securities
7,567,795 2,840 1,033,208 — 6,537,427 
States and political subdivisions
5,030,087 2,316 297,494 — 4,734,909 
Other42,971 — — — 42,971 
Total$16,825,791 $5,156 $1,633,309 $— $15,197,638 
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 $— $16,578,371 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of June 30, 2024, securities available for sale in an unrealized loss position for which an allowance for credit losses has not been recorded, aggregated by type of security and length of time in a continuous unrealized loss position.
Less than 12 MonthsMore than 12 MonthsTotal
Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses		Estimated
Fair Value		Unrealized
Losses
U.S. Treasury$— $— $3,882,331 $302,607 $3,882,331 $302,607 
Residential mortgage-backed securities508,001 5,469 5,561,705 1,027,739 6,069,706 1,033,208 
States and political subdivisions683,217 7,567 3,875,534 289,927 4,558,751 297,494 
Total$1,191,218 $13,036 $13,319,570 $1,620,273 $14,510,788 $1,633,309 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of June 30, 2024. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $43,600 $467,780 $714,399 $1,225,779 
States and political subdivisions9,095 4,273 51,899 2,293,880 2,359,147 
Other1,500 — — — 1,500 
Total$10,595 $47,873 $519,679 $3,008,279 $3,586,426 
Estimated Fair Value
Residential mortgage-backed securities$— $38,538 $416,882 $695,506 $1,150,926 
States and political subdivisions9,068 4,254 49,314 2,201,901 2,264,537 
Other1,474 — — — 1,474 
Total$10,542 $42,792 $466,196 $2,897,407 $3,416,937 
Available For Sale
Amortized Cost
U. S. Treasury$1,173,271 $2,621,114 $197,956 $192,597 $4,184,938 
Residential mortgage-backed securities702 1,118 13,084 7,552,891 7,567,795 
States and political subdivisions176,288 248,998 830,750 3,774,051 5,030,087 
Other— — — — 42,971 
Total$1,350,261 $2,871,230 $1,041,790 $11,519,539 $16,825,791 
Estimated Fair Value
U. S. Treasury$1,152,647 $2,428,473 $163,742 $137,469 $3,882,331 
Residential mortgage-backed securities696 1,121 13,116 6,522,494 6,537,427 
States and political subdivisions175,880 244,313 787,791 3,526,925 4,734,909 
Other— — — — 42,971 
Total$1,329,223 $2,673,907 $964,649 $10,186,888 $15,197,638 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2024202320242023
Proceeds from sales$— $659,082 $— $1,543,355 
Gross realized gains— 561 — 5,417 
Gross realized losses— (528)— (5,363)
Tax (expense) benefit of securities gains/losses— (7)— (11)
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2024202320242023
Premium amortization$(16,570)$(21,242)$(34,523)$(47,167)
Discount accretion5,032 4,843 9,737 10,248 
Net (premium amortization) discount accretion$(11,538)$(16,399)$(24,786)$(36,919)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
June 30,
2024		December 31,
2023
U.S. Treasury$31,400 $30,265 
States and political subdivisions786 1,452 
Total$32,186 $31,717 
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,
2024202320242023
Net gain on sales transactions$1,223 $903 $2,382 $1,871 
Net mark-to-market gains (losses)(82)(10)(101)(27)
Net gain (loss) on trading account securities$1,141 $893 $2,281 $1,844