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Securities (Tables)
 9 Months Ended
Sep. 30, 2021
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, 2021 and December 31, 2020 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
September 30, 2021
Residential mortgage-backed securities
$527,649 $24,332 $— $551,981 $— $527,649 
States and political subdivisions
1,230,279 41,840 32 1,272,087 (158)1,230,121 
Other1,500 — — 1,500 — 1,500 
Total$1,759,428 $66,172 $32 $1,825,568 $(158)$1,759,270 
December 31, 2020
Residential mortgage-backed securities
$528,784 $41,742 $— $570,526 $— $528,784 
States and political subdivisions
1,415,549 65,321 — 1,480,870 (160)1,415,389 
Other1,500 — — 1,500 — 1,500 
Total$1,945,833 $107,063 $— $2,052,896 $(160)$1,945,673 
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, 2021 and December 31, 2020:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
September 30, 2021
Aaa/AAA$95,820 $466,028 $562,320 $1,124,168 $— 
Aa/AA106,111 — — 106,111 — 
A— — — — — 
Not rated— — — — 1,500 
Total$201,931 $466,028 $562,320 $1,230,279 $1,500 
December 31, 2020
Aaa/AAA$115,016 $659,999 $470,745 $1,245,760 $— 
Aa/AA
107,065 — — 107,065 — 
A
62,724 — — 62,724 — 
Not rated— — — — 1,500 
Total$284,805 $659,999 $470,745 $1,415,549 $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, 2021 and 2020.
Three Months Ended
September 30,		Nine Months Ended
September 30,
2021202020212020
Beginning balance$158 $172 $160 $— 
Impact of adopting ASC 326— — — 215 
Credit loss expense (benefit)— (12)(2)(55)
Ending balance$158 $160 $158 $160 
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, 2021 and December 31, 2020 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
September 30, 2021
U.S. Treasury$1,670,692 $20,609 $6,080 $— $1,685,221 
Residential mortgage-backed securities
2,467,915 37,910 14,671 — 2,491,154 
States and political subdivisions
7,049,872 471,883 9,266 — 7,512,489 
Other42,358 — — — 42,358 
Total$11,230,837 $530,402 $30,017 $— $11,731,222 
December 31, 2020
U.S. Treasury$1,084,542 $35,091 $— $— $1,119,633 
Residential mortgage-backed securities
1,916,581 71,102 4 — 1,987,679 
States and political subdivisions
6,683,927 603,975 — — 7,287,902 
Other42,351 — — — 42,351 
Total$9,727,401 $710,168 $4 $— $10,437,565 
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of September 30, 2021, 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$687,535 $6,080 $— $— $687,535 $6,080 
Residential mortgage-backed securities984,236 14,671 — — 984,236 14,671 
States and political subdivisions636,776 9,266 — — 636,776 9,266 
Total$2,308,547 $30,017 $— $— $2,308,547 $30,017 
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, 2021. 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$65 $— $515,356 $12,228 $527,649 
States and political subdivisions456,778 181,302 88,862 503,337 1,230,279 
Other1,500 — — — 1,500 
Total$458,343 $181,302 $604,218 $515,565 $1,759,428 
Estimated Fair Value
Residential mortgage-backed securities$65 $— $539,142 $12,774 $551,981 
States and political subdivisions464,067 187,495 90,295 530,230 1,272,087 
Other1,500 — — — 1,500 
Total$465,632 $187,495 $629,437 $543,004 $1,825,568 
Available For Sale
Amortized Cost
U. S. Treasury$499,857 $533,532 $445,697 $191,606 $1,670,692 
Residential mortgage-backed securities743 16,678 22,146 2,428,348 2,467,915 
States and political subdivisions82,366 1,392,877 863,687 4,710,942 7,049,872 
Other— — — — 42,358 
Total$582,966 $1,943,087 $1,331,530 $7,330,896 $11,230,837 
Estimated Fair Value
U. S. Treasury$501,489 $548,216 $440,328 $195,188 $1,685,221 
Residential mortgage-backed securities772 17,559 22,743 2,450,080 2,491,154 
States and political subdivisions83,932 1,492,026 919,808 5,016,723 7,512,489 
Other— — — — 42,358 
Total$586,193 $2,057,801 $1,382,879 $7,661,991 $11,731,222 
Schedule of Realized Gain (Loss) Sales of securities available for sale were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2021202020212020
Proceeds from sales$— $— $— $1,162,352 
Gross realized gains— — — 108,989 
Gross realized losses— — — — 
Tax (expense) benefit of securities gains/losses— — — (22,888)
Premium Amortization and Discount Accretion Included in Income on Securities [Table Text Block] Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2021202020212020
Premium amortization$(30,240)$(31,540)$(92,016)$(94,873)
Discount accretion657 637 1,999 1,793 
Net (premium amortization) discount accretion$(29,583)$(30,903)$(90,017)$(93,080)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
September 30,
2021		December 31,
2020
U.S. Treasury$24,087 $23,996 
States and political subdivisions1,205 460 
Total$25,292 $24,456 
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,
2021202020212020
Net gain on sales transactions$314 $159 $716 $933 
Net mark-to-market gains (losses)(2)1 (71)94 
Net gain (loss) on trading account securities$312 $160 $645 $1,027