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Acquisition Acquisitions (Details)	3 Months Ended
	Jun. 30, 2018 USD ($) shares
Cobiz Financial Inc. [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Jun. 18, 2018
Cobiz Financial Assets	$ 3,800,000,000
Number of BOKF Shares Given for Each Existing Cobiz Share | shares	0.17
Amount of Cash Paid for Each Exisitng Share of Cobiz Stock	$ 5.70
Switchgrass Holdings LLC [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	May 01, 2018
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred	$ 14,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Goodwill	$ 6,100,000

X

- Definition

Amount of Cash Paid for Each Exisitng Share of Cobiz Stock

+ References

No definition available.

+ Details

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X

- Definition

Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Goodwill

+ References

No definition available.

+ Details

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X

- Definition

Cobiz Financial Assets

+ References

No definition available.

+ Details

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X

- Definition

Number of BOKF Shares Given for Each Existing Cobiz Share

+ References

No definition available.

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X

- Definition

Date when the business acquisition agreement was executed, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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- Definition

Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e845-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e848-128460

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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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.

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No definition available.

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- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to items of consideration transferred in connection with a business combination for which the initial accounting was incomplete.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 4A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=SL65897772-128472

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- Details

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