Loans And Allowance For Loan Losses (Loans Held For Sale, Including Impairment Allowance) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Loans And Allowance For Loan Losses [Abstract]
Student loans, at cost	$ 9,844		$ 28,706
Residential mortgage loans, at cost			2,545
Valuation allowance on student loans	(171)		(175)
Total loans held for sale, at lower of cost or fair value	9,673		31,076
Student loans, net gains on sales	309	68
Residential mortgage loans, net gains on sales	49	503
Total gains on sales of loans held for sale, net	$ 358	$ 571

This element represents the net gains or losses on loans held for sale during the reporting period.

No definition available.

Name:	cbsh_GainsLossesOnSalesOfLoansHeldForSaleNet
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	cbsh_LoansAndAllowanceForLoanLossesAbstract
Namespace Prefix:	cbsh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the carrying value of loans held for sale net of valuation and loan loss allowances.

+ References

No definition available.

+ Details

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

- Definition

This element represents the gain (loss) recognized resulting from the sale of residential mortgage loans held for sales.

+ References

No definition available.

+ Details

Name:	cbsh_ResidentialMortgageLoansRecognizedGainsLosses
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the gain (loss) recognized on student loans held for sale.

+ References

No definition available.

+ Details

Name:	cbsh_StudentLoansRecognizedGainsLosses
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Valuation allowance on student loans held for sale.

+ References

No definition available.

+ Details

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

- Definition

The gross carrying amount of mortgage loans to individuals. This item excludes subordinated loans such as home equity lines of credit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher OTS

-Name Federal Regulation (FR)

-Number Title 12

-Chapter V

-Section 563c.102

-Subsection I

-Paragraph 8

-Subparagraph c(i)

-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6375948&loc=d3e4428-111522

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 01-6

-Paragraph 13

-Subparagraph e

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a)(3),(b))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 7

-Subparagraph a, b

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Chapter 8

-Paragraph 40, 41

-IssueDate 2006-05-01

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

Carrying amount of loans classified as held-for-sale which are offered to individuals to assist in payment of the costs of higher education or qualifying vocational and professional schools. This item represents a broad category of loans including federally subsidized and personal loans. Personal loans include direct and cosigned loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 7

-Subparagraph a

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6375948&loc=d3e4428-111522

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a)(4))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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