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Securities (Tables)
 6 Months Ended
Jun. 30, 2020
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, 2020 and December 31, 2019 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Value		Allowance
for Credit
Losses		Net
Carrying
Amount
June 30, 2020
Residential mortgage-backed securities
$529,868  $45,243  $—  $575,111  $—  $529,868  
States and political subdivisions
1,437,758  59,072  —  1,496,830  (172) 1,437,586  
Other1,500  —  —  1,500  —  1,500  
Total$1,969,126  $104,315  $—  $2,073,441  $(172) $1,968,954  
December 31, 2019
Residential mortgage-backed securities
$530,861  $22  $9,365  $521,518  $—  $530,861  
States and political subdivisions
1,497,644  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 [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, 2020:
States and Political Subdivisions
Not Guaranteed or Pre-RefundedGuaranteed by the Texas PSFPre-RefundedTotalOther
Securities
Aaa/AAA$131,304  $840,004  $292,036  $1,263,344  $—  
Aa/AA
111,427  —  —  111,427  —  
A
62,987  —  —  62,987  —  
Not rated—  —  —  —  1,500  
Total$305,718  $840,004  $292,036  $1,437,758  $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, 2020.
Three Months Ended
June 30, 2020		Six Months Ended June 30, 2020
Beginning balance$200  $—  
Impact of adopting ASC 326—  215  
Credit loss expense (benefit)(28) (43) 
Ending balance$172  $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, 2020 and December 31, 2019 is presented below.
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Allowance
for Credit
Losses		Estimated
Fair Value
June 30, 2020
U.S. Treasury$1,117,867  $45,768  $—  $—  $1,163,635  
Residential mortgage-backed securities
2,051,686  79,604  3  —  2,131,287  
States and political subdivisions
6,725,404  544,091  —  —  7,269,495  
Other42,279  —  —  —  42,279  
Total$9,937,236  $669,463  $3  $—  $10,606,696  
December 31, 2019
U.S. Treasury$1,941,283  $18,934  $12,084  $—  $1,948,133  
Residential mortgage-backed securities
2,176,275  32,608  1,289  —  2,207,594  
States and political subdivisions
6,717,344  353,857  204  —  7,070,997  
Other42,867  —  —  —  42,867  
Total$10,877,769  $405,399  $13,577  $—  $11,269,591  
Securities, with Unrealized Losses Segregated by Length of Impairment
The table below summarizes, as of June 30, 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
$1,243  $2  $127  $1  $1,370  $3  
Total$1,243  $2  $127  $1  $1,370  $3  
Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity The following table summarizes the maturity distribution schedule of securities held to maturity and securities available for sale as of June 30, 2020. Mortgage-backed securities are included in maturity categories based on their stated maturity date. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations. Other securities classified as available for sale include stock in the Federal Reserve Bank and the Federal Home Loan Bank, which have no maturity date. These securities have been included in the total column only.
Within 1 Year1 - 5 Years5 - 10 YearsAfter 10 YearsTotal
Held To Maturity
Amortized Cost
Residential mortgage-backed securities$—  $285  $516,618  $12,965  $529,868  
States and political subdivisions25,717  339,996  425,582  646,463  1,437,758  
Other—  1,500  —  —  1,500  
Total$25,717  $341,781  $942,200  $659,428  $1,969,126  
Estimated Fair Value
Residential mortgage-backed securities$—  $287  $560,606  $14,218  $575,111  
States and political subdivisions25,991  353,852  439,542  677,445  1,496,830  
Other—  1,500  —  —  1,500  
Total$25,991  $355,639  $1,000,148  $691,663  $2,073,441  
Available For Sale
Amortized Cost
U. S. Treasury$33,986  $1,083,881  $—  $—  $1,117,867  
Residential mortgage-backed securities126  57,223  5,424  1,988,913  2,051,686  
States and political subdivisions241,102  411,156  658,064  5,415,082  6,725,404  
Other—  —  —  —  42,279  
Total$275,214  $1,552,260  $663,488  $7,403,995  $9,937,236  
Estimated Fair Value
U. S. Treasury$34,067  $1,129,568  $—  $—  $1,163,635  
Residential mortgage-backed securities133  59,793  5,967  2,065,394  2,131,287  
States and political subdivisions243,001  444,100  714,089  5,868,305  7,269,495  
Other—  —  —  —  42,279  
Total$277,201  $1,633,461  $720,056  $7,933,699  $10,606,696  
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,
2020201920202019
Proceeds from sales$51,250  $2,346,626  $1,162,352  $3,291,529  
Gross realized gains—  803  108,989  803  
Gross realized losses—  (634) —  (634) 
Tax (expense) benefit of securities gains/losses—  (35) (22,888) (35) 
Premium Amortization and Discount Accretion Included in Income on Securities [Table Text Block] Premium amortization and discount accretion included in interest income on securities was as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2020201920202019
Premium amortization$(30,988) $(29,408) $(63,333) $(59,287) 
Discount accretion571  1,378  1,156  2,561  
Net (premium amortization) discount accretion$(30,417) $(28,030) $(62,177) $(56,726) 
Trading Account Securities, at Estimated Fair Value Trading account securities, at estimated fair value, were as follows:
June 30,
2020		December 31,
2019
U.S. Treasury$24,495  $24,298  
Total$24,495  $24,298  
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,
2020201920202019
Net gain on sales transactions$304  $552  $774  $1,058  
Net mark-to-market gains (losses)(22) 27  93  31  
Net gain (loss) on trading account securities$282  $579  $867  $1,089