Fair Value Measurements (Narrative) (Details) (USD $)	Mar. 31, 2012
Fair Value Measurements [Abstract]
Auction rate securities, available for sale	$ 129,900,000
Impairment reserve for student loans held for sale	171,000
Impaired student loan, fair value	$ 6,600,000

Securities by which the interest rate is reset through an auction process.

No definition available.

Name:	cbsh_AuctionRateSecuritiesAvailableForSale
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant