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Securities (Tables)
 9 Months Ended
Sep. 30, 2022
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, 2022 and December 31, 2021 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
September 30, 2022
Residential mortgage-backed securities
$526,397 $— $68,024 $458,373 $— $526,397 
States and political subdivisions
1,646,345 90 188,964 1,457,471 (158)1,646,187 
Other1,500 — 57 1,443 — 1,500 
Total$2,174,242 $90 $257,045 $1,917,287 $(158)$2,174,084 
December 31, 2021
Residential mortgage-backed securities
$527,264 $18,766 $— $546,030 $— $527,264 
States and political subdivisions
1,220,573 41,141 101 1,261,613 (158)1,220,415 
Other1,500 — — 1,500 — 1,500 
Total$1,749,337 $59,907 $101 $1,809,143 $(158)$1,749,179 
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, 2022 and December 31, 2021:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
September 30, 2022
Aaa/AAA$220,180 $1,116,482 $121,042 $1,457,704 $— 
Aa/AA188,641 — — 188,641 — 
Not rated— — — — 1,500 
Total$408,821 $1,116,482 $121,042 $1,646,345 $1,500 
December 31, 2021
Aaa/AAA$92,379 $460,648 $563,251 $1,116,278 $— 
Aa/AA
104,295 — — 104,295 — 
Not rated— — — — 1,500 
Total$196,674 $460,648 $563,251 $1,220,573 $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, 2022 and 2021.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2022202120222021
Beginning balance$158 $158 $158 $160 
Credit loss expense (benefit)— — — (2)
Ending balance$158 $158 $158 $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, 2022 and December 31, 2021 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
September 30, 2022
U.S. Treasury$5,447,478 $— $424,861 $— $5,022,617 
Residential mortgage-backed securities
6,685,418 1,293 987,902 — 5,698,809 
States and political subdivisions
7,121,730 1,395 487,827 — 6,635,298 
Other42,383 — — — 42,383 
Total$19,297,009 $2,688 $1,900,590 $— $17,399,107 
December 31, 2021
U.S. Treasury$2,165,702 $23,333 $9,602 $— $2,179,433 
Residential mortgage-backed securities
4,059,692 31,662 25,089 — 4,066,265 
States and political subdivisions
7,178,135 463,810 5,374 — 7,636,571 
Other42,359 — — — 42,359 
Total$13,445,888 $518,805 $40,065 $— $13,924,628 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of September 30, 2022, 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$4,429,609 $323,346 $593,008 $101,515 $5,022,617 $424,861 
Residential mortgage-backed securities4,280,074 713,685 1,032,311 274,217 5,312,385 987,902 
States and political subdivisions5,943,551 412,938 194,456 74,889 6,138,007 487,827 
Total$14,653,234 $1,449,969 $1,819,775 $450,621 $16,473,009 $1,900,590 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of September 30, 2022. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $— $514,321 $12,076 $526,397 
States and political subdivisions121,042 29,534 9,880 1,485,889 1,646,345 
Other— 1,500 — — 1,500 
Total$121,042 $31,034 $524,201 $1,497,965 $2,174,242 
Estimated Fair Value
Residential mortgage-backed securities$— $— $448,514 $9,859 $458,373 
States and political subdivisions120,786 29,362 9,850 1,297,473 1,457,471 
Other— 1,443 — — 1,443 
Total$120,786 $30,805 $458,364 $1,307,332 $1,917,287 
Available For Sale
Amortized Cost
U. S. Treasury$— $3,821,606 $1,433,912 $191,960 $5,447,478 
Residential mortgage-backed securities28 9,355 17,253 6,658,782 6,685,418 
States and political subdivisions250,317 1,470,384 944,279 4,456,750 7,121,730 
Other— — — — 42,383 
Total$250,345 $5,301,345 $2,395,444 $11,307,492 $19,297,009 
Estimated Fair Value
U. S. Treasury$— $3,648,015 $1,232,969 $141,633 $5,022,617 
Residential mortgage-backed securities27 9,130 17,167 5,672,485 5,698,809 
States and political subdivisions250,875 1,462,732 894,174 4,027,517 6,635,298 
Other— — — — 42,383 
Total$250,902 $5,119,877 $2,144,310 $9,841,635 $17,399,107 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2022202120222021
Premium amortization$(27,165)$(30,240)$(84,278)$(92,016)
Discount accretion4,062 657 7,950 1,999 
Net (premium amortization) discount accretion$(23,103)$(29,583)$(76,328)$(90,017)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
September 30,
2022		December 31,
2021
U.S. Treasury$25,008 $24,237 
States and political subdivisions— 925 
Total$25,008 $25,162 
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,
2022202120222021
Net gain on sales transactions$1,110 $314 $2,462 $716 
Net mark-to-market gains (losses)9 (2)(235)(71)
Net gain (loss) on trading account securities$1,119 $312 $2,227 $645