Derivative Instruments Derivative Instruments (Narrative) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Derivative Instruments (Narrative) [Abstract]
Variation Margin Impact to Positive Fair Values of Cleared Swaps	$ 2.4
Variation Margin Impact to Negative Fair Values of Cleared Swaps	$ 7.3

Variation Margin Impact to Negative Fair Values of Cleared Swaps

No definition available.

Name:	cbsh_VariationMarginImpacttoNegativeFairValuesofClearedSwaps
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant