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Loans And Allowance For Credit Losses (Allowance For Loan Losses And Related Loan Balance Disaggregated On The Basis Of Impairment Methodology) (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Loans And Allowance For Loan Losses [Line Items]
Impaired loans, Allowance for loan losses	$ 2,746
Impaired loans, Loans outstanding	81,732
All other loans, Allowance for loan losses	157,936
All other loans, Loans outstanding	14,656,085
Commercial Portfolio Segment [Member]
Loans And Allowance For Loan Losses [Line Items]
Impaired loans, Allowance for loan losses	1,629
Impaired loans, Loans outstanding	64,500
All other loans, Allowance for loan losses	90,131
All other loans, Loans outstanding	9,233,880
Personal Banking Portfolio Segment [Member]
Loans And Allowance For Loan Losses [Line Items]
Impaired loans, Allowance for loan losses	1,117
Impaired loans, Loans outstanding	17,232
All other loans, Allowance for loan losses	67,805
All other loans, Loans outstanding	$ 5,422,205

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- Definition

Loans And Allowance For Loan Losses [Line Items]

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of allowance for credit losses related to the recorded investment in loans not deemed to be impaired.

+ References

No definition available.

+ Details

Name:	cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Recorded investment of loan receivables, excluding impaired financing receivables.

+ References

No definition available.

+ Details

Name:	cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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