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Securities (Tables)
 12 Months Ended
Dec. 31, 2021
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, 2021 and 2020 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
December 31, 2021
Residential mortgage-backed securities$527,264 $18,766 $— $546,030 $— $527,264 
States and political subdivisions1,220,573 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 
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 
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, 2021:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
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
The following table details activity in the allowance for credit losses on held-to-maturity securities.
20212020
Beginning balance$160 $— 
Impact of adopting ASC 326— 215 
Credit loss expense (benefit)(2)(55)
Ending balance$158 $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, 2021 and 2020 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
December 31, 2021
U.S. Treasury$2,165,702 $23,333 $9,602 $— $2,179,433 
Residential mortgage-backed securities4,059,692 31,662 25,089 — 4,066,265 
States and political subdivisions7,178,135 463,810 5,374 — 7,636,571 
Other42,359 — — — 42,359 
Total$13,445,888 $518,805 $40,065 $— $13,924,628 
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 
Year End Securities with Unrealized Losses, Segregated by Length of Impairment
The table below summarizes, as of December 31, 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
$684,242 $9,602 $— $— $684,242 $9,602 
Residential mortgage-backed securities2,219,261 23,625 45,963 1,464 2,265,224 25,089 
States and political subdivisions
364,056 5,074 5,675 300 369,731 5,374 
Total$3,267,559 $38,301 $51,638 $1,764 $3,319,197 $40,065 
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, 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$37 $— $515,100 $12,127 $527,264 
States and political subdivisions464,112 180,373 73,808 502,280 1,220,573 
Other1,500 — — — 1,500 
Total$465,649 $180,373 $588,908 $514,407 $1,749,337 
Estimated Fair Value
Residential mortgage-backed securities$37 $— $533,454 $12,539 $546,030 
States and political subdivisions468,685 185,458 74,729 532,741 1,261,613 
Other1,500 — — — 1,500 
Total$470,222 $185,458 $608,183 $545,280 $1,809,143 
Available For Sale
Amortized Cost
U. S. Treasury$— $1,030,591 $943,417 $191,694 $2,165,702 
Residential mortgage-backed securities63 15,278 19,175 4,025,176 4,059,692 
States and political subdivisions86,485 1,611,040 808,323 4,672,287 7,178,135 
Other— — — — 42,359 
Total$86,548 $2,656,909 $1,770,915 $8,889,157 $13,445,888 
Estimated Fair Value
U. S. Treasury$— $1,038,734 $942,113 $198,586 $2,179,433 
Residential mortgage-backed securities64 15,954 19,624 4,030,623 4,066,265 
States and political subdivisions87,493 1,715,065 858,485 4,975,528 7,636,571 
Other— — — — 42,359 
Total$87,557 $2,769,753 $1,820,222 $9,204,737 $13,924,628 
Schedule of Realized Gain (Loss) Sales of securities available for sale were as follows:
202120202019
Proceeds from sales$1,999,891 $1,162,352 $18,660,147 
Gross realized gains69 108,989 930 
Gross realized losses— — (637)
Tax benefit (expense) related to securities gains/losses(14)(22,888)(62)
Investment Income Premium amortization and discount accretion included in interest income on securities was as follows:
202120202019
Premium amortization$(121,994)$(126,210)$(120,785)
Discount accretion2,752 2,425 5,227 
Net (premium amortization) discount accretion$(119,242)$(123,785)$(115,558)
Year End Trading Account Securities, at Estimated Fair Value Year-end trading account securities, at estimated fair value, were as follows:
20212020
U.S. Treasury$24,237 $23,996 
States and political subdivisions925 460 
Total$25,162 $24,456 
Net Gains and Losses on Trading Account Securities
Net gains and losses on trading account securities were as follows:
202120202019
Net gain on sales transactions$1,014 $1,102 $2,173 
Net mark-to-market gains (losses)(75)85 (176)
Net gain on trading account securities$939 $1,187 $1,997