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Loans And Allowance For Credit Losses (Collateral-Dependent Loans) (Details) - Commercial Portfolio Segment [Member] $ in Thousands | Dec. 31, 2020 USD ($)
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Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Amortized Cost | $ 16,790
Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business, Amortized Cost | 15,804
Real Estate - Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business Real Estate, Amortized Cost | 986
Business Assets [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Amortized Cost | 13,109
Business Assets [Member] | Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business, Amortized Cost | 13,109
Business Assets [Member] | Real Estate - Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business Real Estate, Amortized Cost | 0
Future Revenue Streams [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Amortized Cost | 986
Future Revenue Streams [Member] | Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business, Amortized Cost | 0
Future Revenue Streams [Member] | Real Estate - Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business Real Estate, Amortized Cost | 986
Energy Service [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Amortized Cost | 2,695
Energy Service [Member] | Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business, Amortized Cost | 2,695
Energy Service [Member] | Real Estate - Business [Member] |
Financing Receivable, Credit Quality Indicator [Line Items] |
Collateral Dependent Loans, Business Real Estate, Amortized Cost | $ 0

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- Definition Collateral Dependent Loans, Amortized Cost + References No definition available. + Details
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Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Collateral Dependent Loans, Business, Amortized Cost + References No definition available. + Details
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Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Collateral Dependent Loans, Business Real Estate, Amortized Cost + References No definition available. + Details
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Namespace Prefix: | cbsh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |