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Borrowings (Narrative) (Details) $ in Millions | Dec. 31, 2020 USD ($)
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Debt Disclosure [Abstract] |
Short-term repurchase agreement comprised of non-insured customer funds | $ 2,100.0
FHLB issued letters of credit | $ 325.5

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- Definition Letters of credit issued by the Federal Home Loan Bank to secure the Company's obligations to certain depositors of public funds. + References No definition available. + Details
Name: | cbsh_FederalHomeLoanBankLettersOfCreditIssued
Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Short-term repurchase agreement comprised of non-insured customer funds + References No definition available. + Details
Name: | cbsh_Shorttermrepurchaseagreementcomprisedofnoninsuredcustomerfunds
Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration