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Loans and Allowance for Credit Losses, Loans by Portfolio Type (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Loan receivables disclosure [Abstract]      
Loans, fixed rate of interest $ 4,478,021 $ 4,529,296 $ 4,557,055
Loans, variable rate of interest 12,679,765 12,436,254 12,227,235
Loans, non-accrual 180,064 187,874 207,616
Total 17,337,850 17,153,424 16,991,906
Accruing loans past due (90 days) [1] 90 633 95
Credit Commitments [Abstract]      
Outstanding commitments to extend credit 10,200,000    
Outstanding standby letters of credit 664,000    
Texas [Member] | Geographic Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 5,900,000    
Loans receivable, other information [Abstract]      
Percentage of loan portfolio attributed to Texas (in hundredths) 34.00%    
Oklahoma [Member] | Geographic Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 3,300,000    
Loans receivable, other information [Abstract]      
Percentage of loan portfolio attributed to Oklahoma (in hundredths) 19.00%    
Commercial      
Loan receivables disclosure [Abstract]      
Loans, fixed rate of interest $ 2,193,469 2,217,432 2,201,040
Loans, variable rate of interest 8,594,738 8,379,240 7,969,245
Loans, non-accrual 131,460 137,303 156,825
Total 10,919,667 10,733,975 10,327,110
Commercial | Texas [Member] | Geographic Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 3,700,000    
Loans receivable, other information [Abstract]      
Percentage of loan portfolio attributed to Texas (in hundredths) 34.00%    
Commercial | Oklahoma [Member] | Geographic Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 1,900,000    
Loans receivable, other information [Abstract]      
Percentage of loan portfolio attributed to Oklahoma (in hundredths) 18.00%    
Commercial | Colorado | Geographic Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 1,000,000    
Loans receivable, other information [Abstract]      
Percentage of loan portfolio attributed to Colorado (in hundredths) 10.00%    
Commercial | Energy [Member]      
Loan receivables disclosure [Abstract]      
Total $ 2,969,618 2,930,156 2,537,112
Commercial | Energy [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 17.00%    
Commercial | Energy Producers [Member] | Credit Concentration Risk [Member]      
Loan receivables disclosure [Abstract]      
Total $ 2,500,000    
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 $ 2,928,294 2,986,949 3,013,375
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 17.00%    
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 2,359,928 2,314,753 2,265,604
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 14.00%    
Commercial real estate      
Loan receivables disclosure [Abstract]      
Loans, fixed rate of interest $ 538,291 548,692 590,375
Loans, variable rate of interest 2,966,021 2,928,440 3,276,213
Loans, non-accrual 2,470 2,855 4,475
Total $ 3,506,782 3,479,987 3,871,063
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) 34.00%    
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 $ 1,587,416 1,608,655 1,616,328
Loans, variable rate of interest 312,559 317,584 283,865
Loans, non-accrual 45,794 47,447 46,081
Total 1,945,769 1,973,686 1,946,274
Residential mortgage | Permanent mortgage [Member]      
Loan receivables disclosure [Abstract]      
Total $ 1,047,785 1,043,435 977,743
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 $ 177,880 197,506 204,181
Residential mortgage | Home equity [Member]      
Loan receivables disclosure [Abstract]      
Total $ 720,104 732,745 764,350
Loans receivable, other information [Abstract]      
Percentage of home equity portfolio comprised of first lien loans 63.00%    
Percentage of home equity portfolio comprised of junior lien loans 37.00%    
Percentage of junior lien home equity loans that are amortizing term loans 46.00%    
Percentage of junior lien home equity loans that are revolving lines of credit 54.00%    
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 $ 158,845 154,517 149,312
Loans, variable rate of interest 806,447 810,990 697,912
Loans, non-accrual 340 269 235
Total $ 965,632 $ 965,776 $ 847,459
[1] Excludes residential mortgage loans guaranteed by agencies of the U.S. government