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Securities (Tables)
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
Debt Securities, Held-to-maturity A summary of the amortized cost, fair value and allowance for credit losses related to securities held to maturity as of December 31, 2020 and 2019 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
December 31, 2020
Residential mortgage-backed securities$528,784 $41,742 $— $570,526 $— $528,784 
States and political subdivisions1,415,549 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 
December 31, 2019
Residential mortgage-backed securities$530,861 $22 $9,365 $521,518 $— $530,861 
States and political subdivisions1,497,644 28,909 896 1,525,657 — 1,497,644 
Other1,500 — — 1,500 — 1,500 
Total$2,030,005 $28,931 $10,261 $2,048,675 $— $2,030,005 
Moody's and Standard & Poor's Bond Ratings
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 December 31, 2020:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
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 $1,500 
Debt Securities, Held-to-maturity, Allowance for Credit Loss
The following table details activity in the allowance for credit losses on held-to-maturity securities during 2020.
Beginning balance$— 
Impact of adopting ASC 326215 
Credit loss expense (benefit)(55)
Ending balance$160 
Debt 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 December 31, 2020 and 2019 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
December 31, 2020
U.S. Treasury$1,084,542 $35,091 $— $— $1,119,633 
Residential mortgage-backed securities1,916,581 71,102 4 — 1,987,679 
States and political subdivisions6,683,927 603,975 — — 7,287,902 
Other42,351 — — — 42,351 
Total$9,727,401 $710,168 $4 $— $10,437,565 
December 31, 2019
U.S. Treasury$1,941,283 $18,934 $12,084 $— $1,948,133 
Residential mortgage-backed securities2,176,275 32,608 1,289 — 2,207,594 
States and political subdivisions6,717,344 353,857 204 — 7,070,997 
Other42,867 — — — 42,867 
Total$10,877,769 $405,399 $13,577 $— $11,269,591 
Year End Securities with Unrealized Losses, Segregated by Length of Impairment
The table below summarizes, as of December 31, 2020, 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
Residential mortgage-backed securities$1,711 $3 $121 $1 $1,832 $4 
Total$1,711 $3 $121 $1 $1,832 $4 
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 December 31, 2020. 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$22 $165 $516,118 $12,479 $528,784 
States and political subdivisions155,941 463,302 207,076 589,230 1,415,549 
Other— 1,500 — — 1,500 
Total$155,963 $464,967 $723,194 $601,709 $1,945,833 
Estimated Fair Value
Residential mortgage-backed securities$22 $167 $556,610 $13,727 $570,526 
States and political subdivisions156,689 480,530 212,937 630,714 1,480,870 
Other— 1,500 — — 1,500 
Total$156,711 $482,197 $769,547 $644,441 $2,052,896 
Available For Sale
Amortized Cost
U. S. Treasury$798,742 $285,800 $— $— $1,084,542 
Residential mortgage-backed securities11,833 25,336 22,583 1,856,829 1,916,581 
States and political subdivisions199,697 1,323,635 638,720 4,521,875 6,683,927 
Other— — — — 42,351 
Total$1,010,272 $1,634,771 $661,303 $6,378,704 $9,727,401 
Estimated Fair Value
U. S. Treasury$811,778 $307,855 $— $— $1,119,633 
Residential mortgage-backed securities12,278 26,724 23,116 1,925,561 1,987,679 
States and political subdivisions201,166 1,442,871 694,760 4,949,105 7,287,902 
Other— — — — 42,351 
Total$1,025,222 $1,777,450 $717,876 $6,874,666 $10,437,565 
Sales of Securities Available for Sale Sales of securities available for sale were as follows:
202020192018
Proceeds from sales$1,162,352 $18,660,147 $16,806,062 
Gross realized gains108,989 930 3 
Gross realized losses— (637)(159)
Tax benefit (expense) related to securities gains/losses(22,888)(62)33 
Premium Amortization and Discount Accretion Included in Income on Securities Premium amortization and discount accretion included in interest income on securities was as follows:
202020192018
Premium amortization$(126,210)$(120,785)$(108,483)
Discount accretion2,425 5,227 7,955 
Net (premium amortization) discount accretion$(123,785)$(115,558)$(100,528)
Year End Trading Account Securities, at Estimated Fair Value Year-end trading account securities, at estimated fair value, were as follows:
20202019
U.S. Treasury$23,996 $24,298 
States and political subdivisions460 — 
Total$24,456 $24,298 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
202020192018
Net gain on sales transactions$1,102 $2,173 $1,816 
Net mark-to-market gains (losses)85 (176)105 
Net gain on trading account securities$1,187 $1,997 $1,921