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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Trust Fees) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Trust Fees [Line Items] |  |  |
Trust fees | $ 160,637 | $ 155,628 | $ 147,964
Private Client [Member] |  |  |
Trust Fees [Line Items] |  |  |
Trust fees | 123,941 | 118,832 | 111,533
Institutional [Member] |  |  |
Trust Fees [Line Items] |  |  |
Trust fees | 30,544 | 29,468 | 29,241
Other Trust Fees [Member] |  |  |
Trust Fees [Line Items] |  |  |
Trust fees | $ 6,152 | $ 7,328 | $ 7,190

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- Definition [Line Items] for Disclosure of information of trust fees earned from contracts with customers + References No definition available. + Details
Name: | cbsh_TrustFeesLineItems
Namespace Prefix: | cbsh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Trust fee revenue + References No definition available. + Details
Name: | cbsh_Trustfeerevenue
Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | cbsh_TrustFeeProductandServicesAxis=cbsh_PrivateClientMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_TrustFeeProductandServicesAxis=cbsh_InstitutionalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_TrustFeeProductandServicesAxis=cbsh_OtherTrustFeesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |