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Fair Value Measurements Fair Value Measurement, Measured On Non-Recurring Basis, Signfiicant Unobservable Inputs, Quantitative Information (Details) - Fair Value, Measurements, Nonrecurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands | 3 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Impaired Loans [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Carrying Value |  | $ 11,763 | $ 3,570 |
Fair Value Measurements, Valuation Techniques |  | Discounted cash flows | Discounted cash flows |
Fair Value Measurements, Significant Unobservable Input |  | Management knowledge of industry and non-real estate collateral including but not limited to recoverable oil and gas reserves, forward-looking commodity prices, estimated operating costs | Recoverable oil and gas reserves, forward-looking commodity prices, estimated operating costs |
Fair Value of Impaired Loans as a Percentage of Unpaid Principal Balance [Abstract] |  |  |  |
Fair Value Measurements, Fair Value of Impaired Loans as a Percentage of Unpaid Principal Balance, Minimum | [1] | 43.00% | 75.00% |
Fair Value Measurements, Fair Value of Impaired Loans as a Percentage of Unpaid Principal Balance, Maximum | [1] | 84.00% | 90.00% |
Fair Value of Impaired Loans as a Percentage of Unpaid Principal Balance, Weighted Average | [1] | 53.00% | 83.00% |
Real estate and other repossessed assets [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Carrying Value |  | $ 6,838 | $ 530 |
Fair Value Measurements, Valuation Techniques |  | Discounted cash flows | Appraised value, as adjusted |
Fair Value Measurements, Significant Unobservable Input |  | Recoverable oil and gas reserves, forward-looking commodity prices, estimated operating costs | Marketability adjustments off appraised value2 | [2]
Fair Value of Real Estate and Other Repossessed Assets as a Percentage of Appraised Value [Abstract] |  |  |  |
Fair Value Measurement, Percentage of Appraised Value. Minimum |  |  | 65.00% |
Fair Value Measurement, Percentage of Appraised Value, Maximum |  |  | 88.00% |
Fair Value Measurement, Percentage of Appraised Value, Weighted Average |  |  | 80.00% |
[1] | Represents fair value as a percentage of the unpaid principal balance.
[2] | Marketability adjustments include consideration of estimated costs to sell which is approximately 10% of the fair value.

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- Definition Represents fair value of other real estate owned as a percentgage of appraised value, high end of range + References No definition available. + Details
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Namespace Prefix: | bokf_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Represents fair value of other real estate owned as a percentgage of appraised value, low end of range + References No definition available. + Details
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- Definition Represents fair value of other real estate owned as a percentgage of appraised value, weighted average + References No definition available. + Details
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Namespace Prefix: | bokf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Description of significant unobservable input used in recurring fair value measurement. + References No definition available. + Details
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- Definition Fair Value of Impaired Loans as a Percentage of Unpaid Principal Balance [Abstract] + References No definition available. + Details
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- Definition Represents the highest percentage in the range of fair values of impaired loans to the unpaid principal balance. + References No definition available. + Details
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- Definition Represents the lowest percentage in the range of fair values of impaired loans to the unpaid principal balance. + References No definition available. + Details
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- Definition Represents the weighted average of the fair values of impaired loans to the unpaid principal balance at the balance sheet date. + References No definition available. + Details
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- Definition Fair Value of Real Estate and Other Repossessed Assets as a Percentage of Appraised Value [Abstract] + References No definition available. + Details
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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
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- 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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- Definition Description of the inputs and valuation technique(s) used to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Details
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