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Securities (Tables)
 3 Months Ended
Mar. 31, 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 March 31, 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
March 31, 2023
Residential mortgage-backed securities
$1,273,079 $3,859 $56,728 $1,220,210 $— $1,273,079 
States and political subdivisions
2,437,262 30,964 88,103 2,380,123 (262)2,437,000 
Other1,500 — 76 1,424 — 1,500 
Total$3,711,841 $34,823 $144,907 $3,601,757 $(262)$3,711,579 
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 March 31, 2023 and December 31, 2022:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFGuaranteed by Third PartyPre-RefundedTotalOther
Securities
March 31, 2023
Aaa/AAA$302,021 $1,557,444 $7,476 $59,123 $1,926,064 $— 
Aa/AA506,432 — 4,766 — 511,198 — 
Not rated— — — — — 1,500 
Total$808,453 $1,557,444 $12,242 $59,123 $2,437,262 $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 months ended March 31, 2023 and 2022.
Three Months Ended
March 31,
20232022
Beginning balance$158 $158 
Credit loss expense (benefit)104 — 
Ending balance$262 $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 March 31, 2023 and December 31, 2022 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
March 31, 2023
U.S. Treasury$5,453,585 $— $325,489 $— $5,128,096 
Residential mortgage-backed securities
7,857,669 13,622 848,349 — 7,022,942 
States and political subdivisions
6,032,658 12,526 256,889 — 5,788,295 
Other42,497 — — — 42,497 
Total$19,386,409 $26,148 $1,430,727 $— $17,981,830 
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 March 31, 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$2,033,194 $44,859 $3,094,902 $280,630 $5,128,096 $325,489 
Residential mortgage-backed securities1,665,095 36,196 4,330,689 812,153 5,995,784 848,349 
States and political subdivisions2,253,851 23,095 1,455,154 233,794 3,709,005 256,889 
Total$5,952,140 $104,150 $8,880,745 $1,326,577 $14,832,885 $1,430,727 
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of March 31, 2023. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$— $— $513,796 $759,283 $1,273,079 
States and political subdivisions72,953 17,842 21,713 2,324,754 2,437,262 
Other— 1,500 — — 1,500 
Total$72,953 $19,342 $535,509 $3,084,037 $3,711,841 
Estimated Fair Value
Residential mortgage-backed securities$— $— $459,002 $761,208 $1,220,210 
States and political subdivisions72,976 17,835 21,257 2,268,055 2,380,123 
Other— 1,424 — — 1,424 
Total$72,976 $19,259 $480,259 $3,029,263 $3,601,757 
Available For Sale
Amortized Cost
U. S. Treasury$1,286,682 $2,539,671 $1,435,094 $192,138 $5,453,585 
Residential mortgage-backed securities8 6,422 17,165 7,834,074 7,857,669 
States and political subdivisions268,607 835,006 853,009 4,076,036 6,032,658 
Other— — — — 42,497 
Total$1,555,297 $3,381,099 $2,305,268 $12,102,248 $19,386,409 
Estimated Fair Value
U. S. Treasury$1,251,908 $2,451,237 $1,275,631 $149,320 $5,128,096 
Residential mortgage-backed securities8 6,340 17,181 6,999,413 7,022,942 
States and political subdivisions269,590 839,354 842,226 3,837,125 5,788,295 
Other— — — — 42,497 
Total$1,521,506 $3,296,931 $2,135,038 $10,985,858 $17,981,830 
Schedule of Realized Gain (Loss) Sales of available for sale securities were as follows:
Three Months Ended
March 31,
20232022
Proceeds from sales$884,273 $— 
Gross realized gains4,856 — 
Gross realized losses(4,835)— 
Tax (expense) benefit of securities gains/losses(4)— 
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
March 31,
20232022
Premium amortization$(25,925)$(29,060)
Discount accretion5,405 1,406 
Net (premium amortization) discount accretion$(20,520)$(27,654)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
March 31,
2023		December 31,
2022
U.S. Treasury$26,076 $25,879 
States and political subdivisions2,659 2,166 
Total$28,735 $28,045 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
Three Months Ended
March 31,
20232022
Net gain on sales transactions$968 $340 
Net mark-to-market gains (losses)(17)(168)
Net gain (loss) on trading account securities$951 $172