Investment Securities (Changes In Recorded Credit Losses) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investment Securities [Abstract]
Balance at January 1	$ 7,542	$ 2,473
Credit losses on debt securities for which impairment was not previously recognized	170	353	3,619
Credit losses on debt securities for which impairment was previously recognized	2,368	4,716
Credit losses reversed on securities sold			(1,146)
Increase in expected cash flows that are recognized over remaining life of security	(149)
Balance at December 31	$ 9,931	$ 7,542	$ 2,473

Cumulative credit losses recognized on impaired debt securities

No definition available.

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- Details

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- Definition

Other-than-temporary impairment credit losses recognized in the reporting period where losses for a particular security were not recognized in a previous reporting period.

+ References

No definition available.

+ Details

Name:	cbsh_LossWhereImpairmentWasNotPreviouslyRecognized
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

Other-than-temporary impairment credit losses recognized in the reporting period where losses for a particular security were recognized in a previous reporting period.

+ References

No definition available.

+ Details

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Balance Type:	debit
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- Definition

Other than temporary impairment credit losses recognized in earnings reversed on securities sold

+ References

No definition available.

+ Details

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

+ References

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 18B

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

-Publisher FASB

-Name Accounting Standards Codification

-Section 50

-Paragraph 8

-Subparagraph (e)

-SubTopic 10

-Topic 320

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27340-111563

+ Details

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