Derivative Instruments Derivative Instruments (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments [Abstract]
Variation Margin Impact to Positive Fair Values of Cleared Swaps	$ 15.9	$ 4.5
Variation Margin Impact to Negative Fair Values of Cleared Swaps	$ 1.5	$ 4.3

Variation Margin Impact to Positive Fair Values of Cleared Swaps

No definition available.

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