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v3.20.4

Stock-Based Compensation and Directors Stock Purchase Plan (Summary Of The Status Of Nonvested Share Awards) (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Nonvested Share Awards [Roll Forward] |  |
Nonvested, share awards, beginning balance | 1,159,180 |
Granted, Shares | 244,468 |
Vested, Shares | (292,100) |
Forfeited, Shares | (11,682) |
Nonvested, share awards, ending balance | 1,099,866 | 1,159,180
Nonvested Weighted Average Grant Date Fair Value [Roll Forward] |  |
Nonvested, Weighted Average Grant Date Fair Value, Beginning Balance | $ 52.11 | $ 45.30
Granted, Weighted Average Grant Date Fair Value |  | 60.90
Vested, Weighted Average Grant Date Fair Value |  | 32.40
Forfeited, Weighted Average Grant Date Fair Value |  | 51.77
Nonvested , Weighted Average Grant Date Fair Value, Ending Balance |  | $ 52.11

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- Definition Nonvested Share Awards, Beginning and Ending Balances + References No definition available. + Details
Name: | cbsh_NonvestedShareAwardsBalances
Namespace Prefix: | cbsh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of nonvested shares that were forfeited during the reporting period. + References No definition available. + Details
Name: | cbsh_NonvestedShareAwardsForfeitedInPeriod
Namespace Prefix: | cbsh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of nonvested shares that were granted during the reporting period. + References No definition available. + Details
Name: | cbsh_NonvestedShareAwardsGrantedInPeriod
Namespace Prefix: | cbsh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of nonvested shares that vested during the reporting period + References No definition available. + Details
Name: | cbsh_NonvestedShareAwardsVestedInPeriod
Namespace Prefix: | cbsh_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Nonvested Share Awards Vested In Period Weighted Average Grant Date Fair Value + References No definition available. + Details
Name: | cbsh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsNonvestedShareAwardsVestedInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix: | cbsh_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Nonvested Share Awards Weighted Average Grant Date Unit Fair Value + References No definition available. + Details
Name: | cbsh_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsNonvestedShareAwardsWeightedAverageGrantDateUnitFairValue
Namespace Prefix: | cbsh_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Share-Based Compensation, Forfeited, Nonvested Share Award, Weighted Average Grant Date Fair Value + References No definition available. + Details
Name: | cbsh_ShareBasedCompensationForfeitedNonvestedShareAwardWeightedAverageGrantDateFairValue
Namespace Prefix: | cbsh_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Definition Share Based Compensation, Granted, Nonvested Share Award Weighted Average Grant Date Fair Value + References No definition available. + Details
Name: | cbsh_ShareBasedCompensationGrantedNonvestedShareAwardWeightedAverageGrantDateFairValue
Namespace Prefix: | cbsh_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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Period Type: | duration

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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