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Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 4,525,904 | $ 4,529,296 | $ 4,540,923
Loans, variable rate of interest |  | 13,312,135 | 12,436,254 | 12,397,283
Loans, non-accrual |  | 165,657 | 187,874 | 245,439
Total |  | 18,003,696 | 17,153,424 | 17,183,645
Accruing loans past due (90 days) | [1] | 879 | 633 | 1,414
Credit Commitments [Abstract] |  |  |  |
Outstanding commitments to extend credit |  | 10,300,000 |  |
Outstanding standby letters of credit |  | 660,000 |  |
Texas [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 6,000,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Texas (in hundredths) |  | 33.00% |  |
Oklahoma [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,500,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Oklahoma (in hundredths) |  | 20.00% |  |
Commercial |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 2,206,735 | 2,217,432 | 2,198,066
Loans, variable rate of interest |  | 9,021,326 | 8,379,240 | 8,242,732
Loans, non-accrual |  | 120,978 | 137,303 | 197,157
Total |  | 11,349,039 | 10,733,975 | 10,637,955
Commercial | Texas [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,800,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Texas (in hundredths) |  | 33.00% |  |
Commercial | Oklahoma [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,200,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Oklahoma (in hundredths) |  | 19.00% |  |
Commercial | Energy [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,147,219 | 2,930,156 | 2,847,240
Commercial | Energy [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,100,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan class to total loans |  | 17.00% |  |
Commercial | Energy Producers [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,600,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of committed energy production loans secured by oil |  | 56.00% |  |
Percentage of committed energy production loans secured by natural gas |  | 44.00% |  |
Commercial | Services [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,944,499 | 2,986,949 | 2,958,827
Commercial | Services [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,900,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan class to total loans |  | 16.00% |  |
Amount of loans with individual balances less than $10 million |  | $ 1,400,000 |  |
Maximum loan amount for certain individual loans in category |  | 10,000 |  |
Commercial | Healthcare [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | 2,353,722 | 2,314,753 | 2,221,518
Commercial | Healthcare [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,400,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan class to total loans |  | 13.00% |  |
Commercial real estate |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 583,782 | 548,692 | 594,542
Loans, variable rate of interest |  | 3,126,442 | 2,928,440 | 3,090,275
Loans, non-accrual |  | 1,996 | 2,855 | 3,775
Total |  | $ 3,712,220 | 3,479,987 | 3,688,592
Commercial real estate | Texas [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 33.00% |  |
Commercial real estate | Oklahoma [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio secured by property in Oklahoma (in hundredths) |  | 12.00% |  |
Residential mortgage |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 1,567,216 | 1,608,655 | 1,597,587
Loans, variable rate of interest |  | 332,691 | 317,584 | 297,376
Loans, non-accrual |  | 42,343 | 47,447 | 44,235
Total |  | 1,942,250 | 1,973,686 | 1,939,198
Residential mortgage | Permanent mortgage [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 1,068,412 | 1,043,435 | 989,040
Loans receivable, other information [Abstract] |  |  |  |
Minimum FICO required for jumbo loan approval |  | 720 |  |
Maximum debt-to-income ratio on jumbo loans (in hundredths) |  | 38.00% |  |
Minimum loan-to-value ratio on jumbo loans (in hundredths) |  | 60.00% |  |
Maximum loan-to-value ratio on jumbo loans (in hundredths) |  | 100.00% |  |
Minimum period for fixed rate on variable rate jumbo loans (in years) |  | 3 years |  |
Maximum period for fixed rate on variable rate jumbo loans (in years) |  | 10 years |  |
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 169,653 | 197,506 | 191,729
Residential mortgage | Home equity [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 704,185 | 732,745 | 758,429
Loans receivable, other information [Abstract] |  |  |  |
Percentage of home equity portfolio comprised of first lien loans |  | 62.00% |  |
Percentage of home equity portfolio comprised of junior lien loans |  | 38.00% |  |
Percentage of junior lien home equity loans that are amortizing term loans |  | 45.00% |  |
Percentage of junior lien home equity loans that are revolving lines of credit |  | 55.00% |  |
Home equity loans, description |  | Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40 percent. |  |
Home equity loans, maximum |  | $ 400 |  |
Home equity loans, revolving period |  | 5 years |  |
Home equity loans, amortization period following revolving period |  | 15 years |  |
Home equity loans, discretionary additional revolving period |  | 5 years |  |
Personal |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 168,171 | 154,517 | 150,728
Loans, variable rate of interest |  | 831,676 | 810,990 | 766,900
Loans, non-accrual |  | 340 | 269 | 272
Total |  | $ 1,000,187 | $ 965,776 | $ 917,900
[1] | Excludes residential mortgage loans guaranteed by agencies of the U.S. government

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