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Loans and Allowances for Credit Losses, Loans by Portfolio Segment (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Loans receivables disclosure [Abstract] |  |  |
Loans receivables with fixed rate of interest |  | $ 6,134,362 | $ 5,749,373
Loans receivables with variable rate of interest |  | 15,435,674 | 15,744,110
Loan receivable non-accrual |  | 180,951 | 163,247
Total |  | 21,750,987 | 21,656,730
Accruing loans past due (90 days) | [1] | 7,680 | 1,338
Foregone interest on nonaccrual loans |  | 17,409 | 15,502
Credit Commitments [Abstract] |  |  |
Outstanding commitments to extend credit |  | 11,100,000 |
Outstanding standby letters of credit |  | 646,000 |
Geographic Concentration Risk [Member] | Texas [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 6,800,000 | $ 6,400,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 31.00% | 30.00%
Geographic Concentration Risk [Member] | Oklahoma [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,500,000 | $ 3,500,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Oklahoma (in hundredths) |  | 16.00% | 16.00%
Geographic Concentration Risk [Member] | Colorado [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 2,800,000 |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Colorado (in hundredths) |  | 13.00% |
Commercial [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Loans receivables with fixed rate of interest |  | $ 3,231,485 | $ 2,251,188
Loans receivables with variable rate of interest |  | 10,684,749 | 11,285,049
Loan receivable non-accrual |  | 115,416 | 99,841
Total |  | 14,031,650 | 13,636,078
Credit Commitments [Abstract] |  |  |
Outstanding commercial letters of credit |  | 1,200 |
Commercial [Member] | Geographic Concentration Risk [Member] | Texas [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 4,700,000 | $ 4,100,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 33.00% | 30.00%
Commercial [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 2,000,000 | $ 2,200,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Oklahoma (in hundredths) |  | 14.00% | 16.00%
Commercial [Member] | Geographic Concentration Risk [Member] | Colorado [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 2,000,000 |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Colorado (in hundredths) |  | 14.00% |
Commercial [Member] | Services [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,122,163 | $ 3,258,192
Commercial [Member] | Services [Member] | Credit Concentration Risk [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,100,000 | $ 3,300,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan class to total loans |  | 14.00% | 15.00%
Amount of loans with individual balances less than $10 million |  | $ 1,500,000 | $ 2,300,000
Maximum loan amount for certain individual loans in category |  | 10,000 | 10,000
Commercial [Member] | Energy [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | 3,973,377 | 3,590,333
Commercial [Member] | Energy [Member] | Credit Concentration Risk [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 4,000,000 | $ 3,600,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan class to total loans |  | 18.00% | 17.00%
Commercial [Member] | Energy Producers [Member] | Credit Concentration Risk [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,100,000 | $ 2,900,000
Loans receivable, other information [Abstract] |  |  |
Percentage of committed energy production loans secured by oil |  | 58.00% | 57.00%
Percentage of committed energy production loans secured by natural gas |  | 42.00% | 43.00%
Commercial [Member] | Healthcare [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,033,916 | $ 2,799,277
Commercial [Member] | Healthcare [Member] | Credit Concentration Risk [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 3,000,000 | $ 2,800,000
Loans receivable, other information [Abstract] |  |  |
Percentage of loan class to total loans |  | 14.00% | 13.00%
Commercial Real Estate [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Loans receivables with fixed rate of interest |  | $ 1,056,321 | $ 1,477,274
Loans receivables with variable rate of interest |  | 3,349,836 | 3,265,918
Loan receivable non-accrual |  | 27,626 | 21,621
Total |  | $ 4,433,783 | $ 4,764,813
Commercial Real Estate [Member] | Geographic Concentration Risk [Member] | Texas [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 24.00% | 26.00%
Commercial Real Estate [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Oklahoma (in hundredths) |  |  | 9.00%
Commercial Real Estate [Member] | Geographic Concentration Risk [Member] | Colorado [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Colorado (in hundredths) |  | 11.00% |
Residential Mortgage [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Loans receivables with fixed rate of interest |  | $ 1,652,653 | $ 1,830,224
Loans receivables with variable rate of interest |  | 393,897 | 358,254
Loan receivable non-accrual |  | 37,622 | 41,555
Total |  | $ 2,084,172 | $ 2,230,033
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Texas [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Texas (in hundredths) |  | 28.00% | 28.00%
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Oklahoma [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Oklahoma (in hundredths) |  | 28.00% | 26.00%
Residential Mortgage [Member] | Geographic Concentration Risk [Member] | Colorado [Member] |  |  |
Loans receivable, other information [Abstract] |  |  |
Percentage of loan portfolio secured by property in Colorado (in hundredths) |  | 15.00% | 19.00%
Residential Mortgage [Member] | Permanent mortgage [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 1,057,321 | $ 1,122,610
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 U.S. government agencies [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | $ 197,794 | 190,866
Residential Mortgage [Member] | Home equity [Member] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Total |  | 829,057 | $ 916,557
Loans receivable, other information [Abstract] |  |  |
Maximum loan amount for certain individual loans in category |  | $ 400 |
Percentage of home equity portfolio comprised of first lien loans |  | 63.00% | 65.00%
Percentage of home equity portfolio comprised of junior lien loans |  | 37.00% | 35.00%
Percentage of junior lien home equity loans that are amortizing term loans |  | 35.00% | 36.00%
Percentage of junior lien home equity loans that are revolving lines of credit |  | 65.00% | 64.00%
Home equity loans, description |  | Home equity loans generally require a minimum FICO score of 700 and a maximum DTI of 40%. |
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] |  |  |
Loans receivables disclosure [Abstract] |  |  |
Loans receivables with fixed rate of interest |  | $ 193,903 | $ 190,687
Loans receivables with variable rate of interest |  | 1,007,192 | 834,889
Loan receivable non-accrual |  | 287 | 230
Total |  | $ 1,201,382 | $ 1,025,806
[1] | Excludes residential mortgage loans guaranteed by agencies of the U.S. government.

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