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Commitments, Contingencies And Guarantees (Schedule Of Off-Balance Sheet Instruments Commitments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Commercial letters of credit	$ 3,117	$ 7,050
Credit Card [Member]
Commitments to extend credit	4,972,104	5,063,166
Other [Member]
Commitments to extend credit	8,033,222	6,123,264
Financial Standby Letter of Credit [Member]
Standby letters of credit, net of participations	$ 357,087	$ 377,338

X

- Definition

Commercial letters of credit.

+ References

No definition available.

+ Details

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

X

- Definition

Commitment To Extend Credit

+ References

No definition available.

+ Details

Name:	cbsh_CommitmentToExtendCredit
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CreditCardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_OtherCreditDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
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