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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Bank Card Transaction Fees) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | $ 151,797 | $ 167,879 | $ 171,576
Debit Card [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 37,644 | 40,025 | 39,738
Debit Card [Member] | Fee income [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 39,862 | 42,106 | 41,522
Debit Card [Member] | Network Charges Expense [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | (2,218) | (2,081) | (1,784)
Credit Card [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 13,393 | 14,177 | 12,965
Credit Card [Member] | Fee income [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 24,921 | 27,416 | 26,799
Credit Card [Member] | Network Charges and Rewards Expense [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | (11,528) | (13,239) | (13,834)
Corporate Card Fees [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 82,374 | 94,388 | 99,640
Corporate Card Fees [Member] | Fee income [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 179,251 | 196,984 | 199,651
Corporate Card Fees [Member] | Network Charges and Rewards Expense [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | (96,877) | (102,596) | (100,011)
Merchant Fees [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 18,386 | 19,289 | 19,233
Merchant Fees [Member] | Fee income [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | 29,660 | 31,517 | 30,241
Merchant Fees [Member] | Network Charges Expense [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | (3,159) | (3,449) | (3,177)
Merchant Fees [Member] | Fees to Cardholder Banks [Member] |  |  |
Bank Card Transaction Fees [Line Items] |  |  |
Bank card transaction fees | $ (8,115) | $ (8,779) | $ (7,831)

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- Definition Fees earned from the issuance of debt and credit cards to the Company's retail and commercial banking customers who use the cards to purchase goods and services from merchants through an electronic payment system. + References No definition available. + Details
Name: | cbsh_BankCardTransactionFees
Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Details
Name: | cbsh_BankCardProductandServicesAxis=us-gaap_DebitCardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_BankCardTransactionsAxis=cbsh_FeeincomeMember
Namespace Prefix: |
Data Type: | na
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Period Type: |

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- Details
Name: | cbsh_BankCardTransactionsAxis=cbsh_NetworkChargesExpenseMember
Namespace Prefix: |
Data Type: | na
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Period Type: |

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- Details
Name: | cbsh_BankCardProductandServicesAxis=us-gaap_CreditCardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_BankCardTransactionsAxis=cbsh_NetworkChargesandRewardsExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cbsh_BankCardProductandServicesAxis=cbsh_CorporateCardFeesMember
Namespace Prefix: |
Data Type: | na
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- Details
Name: | cbsh_BankCardProductandServicesAxis=cbsh_MerchantFeesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
Name: | cbsh_BankCardTransactionsAxis=cbsh_FeestoCardholderBanksMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |