Common Stock Common Stock (Accelerated Share Repurchase) (Details) - USD ($) $ in Millions	Aug. 27, 2015	Jun. 24, 2015	May. 21, 2015	Jun. 20, 2014
Common Stock (Accelerated Share Repurchase) [Abstract]
Accelerated Share Repurchase Program Repurchase Amount			$ 100.0	$ 200.0
Shares received under accelerated share repurchase program	351,620	1,480,378	1,803,427	3,208,206

Shares received to date under accelerated share repurchase program.

No definition available.

Name:	cbsh_Sharesreceivedunderacceleratedsharerepurchaseprogram
Namespace Prefix:	cbsh_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration