Loans and Allowance for Loan Losses (Loans Held for Sale, Including Impairment Losses) (Details) (USD $) In Thousands	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Loans and Allowance for Loan Losses
Student loans, at cost	$ 37,315		$ 53,901
Residential mortgage loans, at cost	2,437		10,419
Valuation allowance on student loans	(176)		(569)
Total loans held for sale at lower of cost or fair value	39,576		63,751
Student loans, net gains	382	5,347
Residential mortgage loans, net gains	1,172	1,309
Total gains on sales of loans held for sale, net	$ 1,554	$ 6,656

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

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

- Definition

Loans and Allowance for Loan Losses

+ 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.

+ 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.

+ 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.

+ 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.

+ 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

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant