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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Contract with Customer, Asset and Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Bank Card Transaction Fees [Member]
Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Receivable	$ 14,199	$ 13,915	$ 13,035
Trust Fees [Member]
Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Receivable	2,071	2,093	2,721
Deposit Account Charges and Other Fees [Member]
Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Receivable	6,933	6,523	6,107
Consumer Brokerage Services [Member]
Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Receivable	$ 432	$ 596	$ 559

X

- Definition

[Line Items] for Disclosure of information about accounts receivable due from contracts with customers.

+ References

No definition available.

+ Details

Name:	cbsh_ContractwithCustomerAssetandLiabilityLineItems
Namespace Prefix:	cbsh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount receivable in exchange for services transferred to customers that is unconditional.

+ References

No definition available.

+ Details

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

X

- Details

Name:	srt_ProductOrServiceAxis=cbsh_BankCardTransactionFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_ProductOrServiceAxis=cbsh_TrustFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_ProductOrServiceAxis=cbsh_DepositAccountChargesandOtherFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_ProductOrServiceAxis=cbsh_ConsumerBrokerageServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: