Revenue from Contracts with Customers Revenue from Contracts with Customers (Narrative) (Details)	3 Months Ended
Mar. 31, 2018 Rate
Revenue from Contract with Customer [Abstract]
Percent of Revenue not in scope of ASC 606	62.00%

Percent of total revenue not within the scope of ASC 606

No definition available.

Name:	cbsh_PercentofRevenuenotinscopeofASC606
Namespace Prefix:	cbsh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration