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

X

- 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

X

- 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

X

- References

No definition available.

+ Details

Name:	us-gaap_DebtDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration