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Revenue from Contracts with Customers Revenue from Contracts with Customers (Narrative) (Details)	12 Months Ended
	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

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- References

No definition available.

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration