Loans And Allowance For Credit Losses (Average Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2019
Financing Receivable, Impaired [Line Items]
Non-accrual loans	$ 11,558	$ 11,955
Restructured loans (accruing)	69,161	63,853
Average Impaired Loans, Total	80,719	75,808
Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	9,655	9,881
Restructured loans (accruing)	53,517	48,248
Average Impaired Loans, Total	63,172	58,129
Personal Banking Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Non-accrual loans	1,903	2,074
Restructured loans (accruing)	15,644	15,605
Average Impaired Loans, Total	$ 17,547	$ 17,679

Average recorded investment in financing receivables on Nonaccrual status.

No definition available.

Name:	cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Average recorded investment in financing receivables modified in a troubled debt restructuring.

+ References

No definition available.

+ Details

Name:	cbsh_FinancingReceivableModificationsAverageRecordedInvestment
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Average 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 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5258-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbsh_PersonalBankingPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: