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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
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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

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- 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

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- 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

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

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

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

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