Employee Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	3.32%
Quarterly variable interest percentage accrued on employee account balances, maximum	3.40%
Defined Benefit Plan, Benefit Obligation	$ 24,500,000	$ 24,500,000
Defined Benefit Plan, Plan Assets, Amount	36,400,000	33,600,000
Net periodic benefit cost (credit)	(815,000)	$ (583,000)	$ (704,000)
Total estimated future benefit payments	$ 27,800,000
BOK Financial Pension Plan Weighted-average Assumptions [Abstract]
Discount rate (in hundredths)	4.10%	3.30%
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 years
Defined Contribution Plan, Maximum Years of Service for Employees to Obtain Minimum Employer Matching	15 years
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	5 years
Defined Contribution Plan, Cost	$ 27,600,000	$ 25,100,000	$ 22,800,000
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%