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Goodwill And Other Intangible Assets (Schedule Of Intangible Assets With Estimable Useful Lives) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount $ 35,114us-gaap_FiniteLivedIntangibleAssetsGross $ 34,963us-gaap_FiniteLivedIntangibleAssetsGross
Accumulated Amortization (27,889)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization (27,416)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Valuation Allowance (82)us-gaap_ValuationAllowancesAndReservesBalance (97)us-gaap_ValuationAllowancesAndReservesBalance
Net Amount 7,143us-gaap_IntangibleAssetsNetExcludingGoodwill 7,450us-gaap_IntangibleAssetsNetExcludingGoodwill
Core Deposit Premium [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 31,270us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
31,270us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
Accumulated Amortization (25,121)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
(24,698)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
Valuation Allowance 0us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
0us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
Net Amount 6,149us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
6,572us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
Mortgage Servicing Rights [Member]    
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 3,844us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
3,693us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
Accumulated Amortization (2,768)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
(2,718)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
Valuation Allowance (82)us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
(97)us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
Net Amount $ 994us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
$ 878us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember