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Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 4,508,969 | $ 4,529,296 | $ 4,532,914
Loans, variable rate of interest |  | 13,687,498 | 12,436,254 | 12,448,284
Loans, non-accrual |  | 152,992 | 187,874 | 225,636
Total |  | 18,349,459 | 17,153,424 | 17,206,834
Accruing loans past due (90 days) | [1] | 518 | 633 | 253
Credit Commitments [Abstract] |  |  |  |
Outstanding commitments to extend credit |  | 10,700,000 |  |
Outstanding standby letters of credit |  | 672,000 |  |
Texas [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 6,100,000 |  |
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 |  | $ 3,600,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Oklahoma (in hundredths) |  | 20.00% |  |
Commercial [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 2,150,138 | 2,217,432 | 2,225,470
Loans, variable rate of interest |  | 9,316,473 | 8,379,240 | 8,393,564
Loans, non-accrual |  | 109,490 | 137,303 | 176,900
Total |  | 11,576,101 | 10,733,975 | 10,795,934
Commercial [Member] | 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 [Member] | Oklahoma [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,300,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio attributed to Oklahoma (in hundredths) |  | 20.00% |  |
Commercial [Member] | Energy [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,294,867 | 2,930,156 | 2,867,981
Commercial [Member] | Energy [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,300,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan class to total loans |  | 18.00% |  |
Commercial [Member] | Energy Producers [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 2,700,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of committed energy production loans secured by oil |  | 57.00% |  |
Percentage of committed energy production loans secured by natural gas |  | 43.00% |  |
Commercial [Member] | Services [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,017,311 | 2,986,949 | 2,967,513
Commercial [Member] | Services [Member] | Credit Concentration Risk [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 3,000,000 |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan class to total loans |  | 16.00% |  |
Amount of loans with individual balances less than $10 million |  | $ 1,500,000 |  |
Maximum loan amount for certain individual loans in category |  | 10,000 |  |
Commercial [Member] | Healthcare [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | 2,437,323 | 2,314,753 | 2,239,451
Commercial [Member] | 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 [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 603,515 | 548,692 | 564,681
Loans, variable rate of interest |  | 3,199,844 | 2,928,440 | 2,950,486
Loans, non-accrual |  | 1,316 | 2,855 | 2,975
Total |  | $ 3,804,675 | 3,479,987 | 3,518,142
Commercial real estate [Member] | Texas [Member] | Geographic Concentration Risk [Member] |  |  |  |
Loans receivable, other information [Abstract] |  |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 32.00% |  |
Commercial real estate [Member] | 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 [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 1,592,249 | 1,608,655 | 1,589,013
Loans, variable rate of interest |  | 337,576 | 317,584 | 311,231
Loans, non-accrual |  | 41,917 | 47,447 | 45,506
Total |  | 1,971,742 | 1,973,686 | 1,945,750
Residential mortgage [Member] | Permanent mortgage [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 1,094,926 | 1,043,435 | 1,013,965
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 [Member] | Permanent mortgages guaranteed by US government agencies [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 180,718 | 197,506 | 187,370
Residential mortgage [Member] | Home equity [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Total |  | $ 696,098 | 732,745 | 744,415
Loans receivable, other information [Abstract] |  |  |  |
Percentage of home equity portfolio comprised of first lien loans |  | 61.00% |  |
Percentage of home equity portfolio comprised of junior lien loans |  | 39.00% |  |
Percentage of junior lien home equity loans that are amortizing term loans |  | 44.00% |  |
Percentage of junior lien home equity loans that are revolving lines of credit |  | 56.00% |  |
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 [Member] |  |  |  |
Loan receivables disclosure [Abstract] |  |  |  |
Loans, fixed rate of interest |  | $ 163,067 | 154,517 | 153,750
Loans, variable rate of interest |  | 833,605 | 810,990 | 793,003
Loans, non-accrual |  | 269 | 269 | 255
Total |  | $ 996,941 | $ 965,776 | $ 947,008
[1] | Excludes residential mortgage loans guaranteed by agencies of the U.S. government

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