Loans and Allowance for Loan Losses (Loans Held for Sale, Including Impairment Losses) (Details) (USD $) In Thousands	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Loans and Allowance for Loan Losses
Student loans, at cost	$ 38,079		$ 53,901
Residential mortgage loans, at cost	4,459		10,419
Valuation allowance on student loans	(179)		(569)
Total loans held for sale at lower of cost or fair value	42,359		63,751
Student loans, net gains	386	1,689
Residential mortgage loans, net gains	761	777
Total gains on sales of loans held for sale, net	$ 1,147	$ 2,466

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

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

- Definition

Loans and Allowance for Loan Losses

+ Details

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Data Type:	xbrli:stringItemType
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- Definition

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

+ Details

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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
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Reflects the gross carrying amount of unpaid loans issued to individuals to acquire or improve land or a building or to provide cash to the borrower, and which are secured by the acquired or improved realty. Loans secured by one to four-family residential property of the borrower are generally referred to as residential mortgage loans. Repayment terms for residential mortgage loans may vary considerably. Such loans may be structured to provide for full amortization of principal, partial amortization with a balloon payment at a specified date, or negative amortization. Interest rates may be fixed or variable. This item excludes subordinated loans such as home equity lines of credit.

+ References

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Statement of Position (SOP)
-Number 01-6
-Paragraph 13
-Subparagraph e

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 7
-Subparagraph a, b
-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher OTS
-Name Federal Regulation (FR)
-Number Title 12
-Chapter V
-Section 563c.102
-Paragraph 8
-Subparagraph c(i)
-Subsection I

+ Details

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

- Definition

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
-Section 03
-Paragraph 7
-Subparagraph a
-Article 9

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant