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Resale and Repurchase Agreements (Narrative) (Details) - Collateral Swap [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Collateral swap agreements	$ 200.0	$ 200.0
Collateral Already Posted, Aggregate Fair Value	206.9
Collateral Accepted, Aggregate Fair Value	$ 209.0

X

- Definition

Collateral Accepted, Aggregate Fair Value

+ References

No definition available.

+ Details

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

X

- Definition

Collateral Swap Agreements

+ References

No definition available.

+ Details

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

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

+ Details

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

X

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cbsh_CollateralSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: