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Employee Benefits (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
BOK Financial Pension Plan [Member]      
BOK Financial Pension Plan [Abstract]      
Interest percentage added to the five-year trailing average of five-year Treasury Securities 1.50%    
Interest percentage accrued on employee account balances, Minimum 3.00%    
Interest percentage accrued on employee account balances, Maximum 5.00%    
Quarterly variable interest percentage accrued on employee account balances, minimum 3.28%    
Quarterly variable interest percentage accrued on employee account balances, maximum 3.40%    
Defined Benefit Plan, Benefit Obligation $ 24,000,000 $ 25,000,000  
Defined Benefit Plan, Plan Assets, Amount 38,000,000 36,000,000  
Net periodic benefit cost (credit) (1,300,000) $ (815,000) $ (583,000)
Total estimated future benefit payments $ 26,600,000    
BOK Financial Pension Plan Weighted-average Assumptions [Abstract]      
Discount rate (in hundredths) 2.69% 4.10%  
Expected return on plan assets (in hundredths) 5.50% 5.50%  
BOK Financial Pension Plan [Member] | Equity Securities [Member]      
BOK Financial Pension Plan [Abstract]      
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 60.00%    
BOK Financial Pension Plan [Member] | Bonds [Member]      
BOK Financial Pension Plan [Abstract]      
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage 40.00%    
BOK Financial 401(k) Plan [Member]      
BOK Financial 401(k) Plan [Abstract]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 6.00%    
Defined Contribution Plan, Minimum Years of Service for Employees to Obtain Maximum Employer Matching 4    
Defined Contribution Plan, Maximum Years of Service for Employees to Obtain Minimum Employer Matching 15    
Contributions Non-Elective Annual Contributions For Qualified Employees $ 750    
Defined Contribution Plan, Annual Base Employee Compensation to Qualify for Non-Elective Employer Contributions, Maximum $ 40,000    
Defined Contribution Plan, Vesting Period for Employer Contributions five years    
Defined Contribution Plan, Cost $ 29,900,000 $ 27,600,000 $ 25,100,000
BOK Financial 401(k) Plan [Member] | Minimum [Member]      
BOK Financial 401(k) Plan [Abstract]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 50.00%    
BOK Financial 401(k) Plan [Member] | Maximum [Member]      
BOK Financial 401(k) Plan [Abstract]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 200.00%