Loans and Allowances for Credit Losses, Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructuring, Recorded Investment	$ 159		$ 166
Troubled Debt Restructuring, Performing in Accordance With Modified Terms	81		71
Financing Receivable, Modifications, Post-Modification Recorded Investment	18	$ 37
Permanent mortgages guaranteed by US government agencies [Member] \| Accruing [Member] \| Residential mortgage [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructuring, Recorded Investment	$ 92		$ 86

Represents the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructuring and are performing according to modified terms.

No definition available.

+ Details

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Namespace Prefix:	bokf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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- Details

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- Details

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