Significant Accounting Policies Loans and Allowance for Credit Losses (Details)	9 Months Ended
Sep. 30, 2020
Loans and Allowance for Credit Losses [Abstract]
Loans, Number of Days Past Due for a Non-Risk Graded Loan to be Placed on Nonaccruing Status	90 days
Loans, Number of Days After Notification of Chapter 7 Bankruptcy Non-Risk Graded Loan is Placed on Nonaccruing Status	60 days
Loans and Allowances for Credit Losses, Minimum No of Days After Which Past Due Non-Risk Graded Loans Are Charged Off	60 days
Loans and Allowances for Credit Losses, Maximum No of Days After Which Past Due Non-Risk Graded Loans Are Charged Off	180 days
Loans, Number of Days After Notification of Chapter 7 Bankruptcy Non-Risk Graded Loan is Charged Off	60 days

Loans and Allowance for Credit Losses [Abstract]

No definition available.

Name:	bokf_LoansAndAllowanceForCreditLossesAbstract
Namespace Prefix:	bokf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents the number of days after notification of Chapter 7 bankruptcy for a non-risk graded loan to be charged off

No definition available.

Name:	bokf_LoansNumberOfDaysAfterNotificationOfChapter7BankruptcyNonRiskGradedLoanIsChargedOff
Namespace Prefix:	bokf_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

The number of days following notification of a borrowers Chapter 7 bankruptcy that a non-risk graded loan is placed on nonaccruing status.

No definition available.

Name:	bokf_LoansNumberOfDaysAfterNotificationOfChapter7BankruptcyNonRiskGradedLoanIsPlacedOnNonaccruingStatus
Namespace Prefix:	bokf_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

Represents the number of days past due a non-risk graded loan has to be in order to be placed on nonaccruing status.

No definition available.

Name:	bokf_LoansNumberOfDaysPastDueForNonRiskGradedLoanToBePlacedOnNonaccruingStatus
Namespace Prefix:	bokf_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

Maximum number of days after which past due non-risk graded loans are charged off

No definition available.

Name:	bokf_MaxNoOfDaysAfterWhichPastDueNonRiskGradedLoansAreChargedOff
Namespace Prefix:	bokf_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

Minimum number of days after which past due non-risk graded loans are charged off

No definition available.

Name:	bokf_MinimumNoOfDaysAfterWhichPastDueNonRiskGradedLoansAreChargedOff
Namespace Prefix:	bokf_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration