Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 09, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Type	10-K
Amendment Flag	false
Document Fiscal Period Focus	FY
Entity Registrant Name	COMMERCE BANCSHARES INC /MO/
Entity Central Index Key	0000022356
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		106,710,134
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 5,271,000,000