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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Deposit Account Charges and Other Fees) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Deposit Account Charges and Other Fees [Line Items] |  |  |
Deposit account charges and other fees | $ 93,227 | $ 95,983 | $ 94,517
Corporate Cash Management [Member] |  |  |
Deposit Account Charges and Other Fees [Line Items] |  |  |
Deposit account charges and other fees | 46,762 | 41,442 | 38,468
Overdraft and Return Item [Member] |  |  |
Deposit Account Charges and Other Fees [Line Items] |  |  |
Deposit account charges and other fees | 22,951 | 30,596 | 31,468
Other Deposit Account Charges [Member] |  |  |
Deposit Account Charges and Other Fees [Line Items] |  |  |
Deposit account charges and other fees | $ 23,514 | $ 23,945 | $ 24,581

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

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

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- Details
Name: | cbsh_DepositaccountfeeproductandservicesAxis=cbsh_CorporateCashManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_DepositaccountfeeproductandservicesAxis=cbsh_OverdraftandReturnItemMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_DepositaccountfeeproductandservicesAxis=cbsh_OtherDepositAccountChargesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |