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v3.20.4

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

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- 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

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- 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

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