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Securities (Tables)
 6 Months Ended
Jun. 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 June 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
June 30, 2021
Residential mortgage-backed securities
$527,958 $30,129 $1 $558,086 $— $527,958 
States and political subdivisions
1,284,105 52,544 — 1,336,649 (158)1,283,947 
Other1,500 — — 1,500 — 1,500 
Total$1,813,563 $82,673 $1 $1,896,235 $(158)$1,813,405 
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 June 30, 2021 and December 31, 2020:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
June 30, 2021
Aaa/AAA$101,978 $478,728 $534,514 $1,115,220 $— 
Aa/AA106,429 — — 106,429 — 
A62,456 — — 62,456 — 
Not rated— — — — 1,500 
Total$270,863 $478,728 $534,514 $1,284,105 $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 six months ended June 30, 2021 and 2020.
Three Months Ended
June 30,		Six Months Ended
June 30,
2021202020212020
Beginning balance$158 $200 $160 $— 
Impact of adopting ASC 326— — — 215 
Credit loss expense (benefit)— (28)(2)(43)
Ending balance$158 $172 $158 $172 
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, 2021 and December 31, 2020 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
June 30, 2021
U.S. Treasury$1,473,800 $26,664 $2,742 $— $1,497,722 
Residential mortgage-backed securities
1,729,198 40,589 8,158 — 1,761,629 
States and political subdivisions
7,014,119 526,628 1,928 — 7,538,819 
Other42,356 — — — 42,356 
Total$10,259,473 $593,881 $12,828 $— $10,840,526 
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 June 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$194,344 $2,742 $— $— $194,344 $2,742 
Residential mortgage-backed securities673,043 8,158 — — 673,043 8,158 
States and political subdivisions150,945 1,928 — — 150,945 1,928 
Total$1,018,332 $12,828 $— $— $1,018,332 $12,828 
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, 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$48 $54 $515,611 $12,245 $527,958 
States and political subdivisions260,563 337,281 116,087 570,174 1,284,105 
Other1,500 — — — 1,500 
Total$262,111 $337,335 $631,698 $582,419 $1,813,563 
Estimated Fair Value
Residential mortgage-backed securities$48 $55 $544,894 $13,089 $558,086 
States and political subdivisions264,142 348,160 118,396 605,951 1,336,649 
Other1,500 — — — 1,500 
Total$265,690 $348,215 $663,290 $619,040 $1,896,235 
Available For Sale
Amortized Cost
U. S. Treasury$799,521 $285,676 $197,086 $191,517 $1,473,800 
Residential mortgage-backed securities3,453 19,270 22,748 1,683,727 1,729,198 
States and political subdivisions129,046 1,367,342 817,906 4,699,825 7,014,119 
Other— — — — 42,356 
Total$932,020 $1,672,288 $1,037,740 $6,575,069 $10,259,473 
Estimated Fair Value
U. S. Treasury$804,722 $302,656 $194,344 $196,000 $1,497,722 
Residential mortgage-backed securities3,592 20,327 23,379 1,714,331 1,761,629 
States and political subdivisions130,265 1,473,655 875,528 5,059,371 7,538,819 
Other— — — — 42,356 
Total$938,579 $1,796,638 $1,093,251 $6,969,702 $10,840,526 
Sales of Securities Available for Sale Sales of securities available for sale were as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2021202020212020
Proceeds from sales$— $51,250 $— $1,162,352 
Gross realized gains— — — 108,989 
Gross realized losses— — — — 
Tax (expense) benefit of securities gains/losses— — — (22,888)
Premium Amortization and Discount Accretion Included in Income on Securities [Table Text Block] Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2021202020212020
Premium amortization$(30,541)$(30,988)$(61,776)$(63,333)
Discount accretion705 571 1,342 1,156 
Net (premium amortization) discount accretion$(29,836)$(30,417)$(60,434)$(62,177)
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
June 30,
2021		December 31,
2020
U.S. Treasury$24,137 $23,996 
States and political subdivisions— 460 
Total$24,137 $24,456 
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,
2021202020212020
Net gain on sales transactions$293 $304 $402 $774 
Net mark-to-market gains (losses)— (22)(69)93 
Net gain (loss) on trading account securities$293 $282 $333 $867