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Revenue from Contracts with Customers Revenue from Contracts with Customers (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2020 Rate
Revenue from Contract with Customer [Abstract] |
Percent of Revenue not in scope of ASC 606 | 62.00%

X
- Definition Percent of total revenue not within the scope of ASC 606 + References No definition available. + Details
Name: | cbsh_PercentofRevenuenotinscopeofASC606
Namespace Prefix: | cbsh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration