Other Comprehensive Income (Schedule of Calculation of Other Comprehensive Income) (Details) (USD $) In Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Amortization of accumulated pension loss	$ 540	$ 567	$ 1,080	$ 1,134
Net unrealized gains (losses)			15,474	21,267
Net unrealized gains			3,511	7,420
Other comprehensive income	21,866	17,709	19,655	29,390
Available for Sale Debt Securities for Which a Portion of OTTI Has Been Recorded in Earnings [Member]
Unrealized holding gains (losses) subsequent to initial OTTI recognition	(812)	5,665	5,663	11,967
Income tax (expense) benefit	309	(2,153)	(2,152)	(4,547)
Net unrealized gains (losses)	(503)	3,512	3,511	7,420
Other Available for Sale Investment Securities [Member]
Unrealized holding gains (losses) subsequent to initial OTTI recognition	35,540	23,693	25,135	36,073
Reclassification adjustment for gains included in net income	(1)	(1,362)	(177)	(1,770)
Net unrealized gains on securities	35,539	22,331	24,958	34,303
Income tax (expense) benefit	(13,505)	(8,486)	(9,484)	(13,036)
Net unrealized gains	22,034	13,845	15,474	21,267
Prepaid Pension Cost [Member]
Amortization of accumulated pension loss	540	567	1,080	1,134
Income tax (expense) benefit	(205)	(215)	(410)	(431)
Pension loss amortization	$ 335	$ 352	$ 670	$ 703

Appreciation or loss in value (net of reclassification adjustment) of the total of unsold securities (excluding OTTI securities) during the period being reported on, net of tax. Reclassification adjustments include: (1) the unrealized holding gain or loss, net of tax, at the date of the transfer for a debt security from the held-to-maturity category transferred into the available-for-sale category. Also includes the unrealized gain or loss at the date of transfer for a debt security from the available-for-sale category transferred into the held-to-maturity category; (2) the unrealized gains or losses realized upon the sale of securities, after tax.

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

- Definition

Amortization of defined benefit plan accumulated pension loss, net of tax

+ Details

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

- Definition

Unrealized gains and losses recognized in Other Comprehensive Income on available for sale debt securities for which a portion of an other-than-temporary impairment has been recognized in earnings, after tax.

+ Details

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

- Definition

The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the reporting entity. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, but excludes any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph A5
-Appendix A

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 30

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 38
-Subparagraph c(3)

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 8, 9, 10, 11, 12, 13, 14

+ Details

Name:	us-gaap_ComprehensiveIncomeNetOfTax
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amounts in accumulated other comprehensive income related to gains and losses that are not recognized immediately and are expected to be recognized as components of net periodic benefit cost over the next fiscal year that follows the most recent annual statement of financial position presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 5
-Subparagraph s

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 158
-Paragraph 7
-Subparagraph d

+ Details

Name:	us-gaap_DefinedBenefitPlanAmortizationOfNetGainsLosses
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre tax reclassification adjustment for gains or losses realized upon the sale of securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 38
-Subparagraph c(3)

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 130
-Paragraph 24
-Subparagraph b

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 130
-Paragraph 18, 19

+ Details

Name:	us-gaap_OtherComprehensiveIncomeReclassificationAdjustmentForSaleOfSecuritiesIncludedInNetIncomeBeforeTax
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Gross appreciation or the gross loss in value of the total of unsold available-for-sale securities during the period being reported (except when designated as a fair value hedge).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 130
-Paragraph 24
-Subparagraph b

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
-Paragraph 38
-Subparagraph c(3)

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 130
-Paragraph 17

+ Details

Name:	us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Tax effect on gross appreciation or the gross loss in value of the total of unsold securities during the period being reported on.

+ References

+ Details

Name:	us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increases (decreases) in the market value of unsold securities whose gains (losses) were included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 95
-Paragraph 28

+ Details

Name:	us-gaap_UnrealizedGainLossOnSecurities
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration