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Goodwill And Other Intangible Assets (Schedule Of Changes In Carrying Amount Of Goodwill And Net Other Intangible Assets) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Goodwill and Intangible Assets [Roll Forward]      
Amortization $ (473)us-gaap_AmortizationOfIntangibleAssets $ (559)us-gaap_AmortizationOfIntangibleAssets  
Goodwill [Member]      
Goodwill and Intangible Assets [Roll Forward]      
Originations 0us-gaap_ServicingAssetAtFairValueAdditions
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_GoodwillMember
   
Amortization 0us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_GoodwillMember
   
Impairment reversal 0cbsh_ImpairmentAndImpairmentReversalsOfIntangibleAssetsFiniteLived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_GoodwillMember
   
Intangible Assets, Net (Including Goodwill) 138,921us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_GoodwillMember
  138,921us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_GoodwillMember
Core Deposit Premium [Member]      
Goodwill and Intangible Assets [Roll Forward]      
Originations 0us-gaap_ServicingAssetAtFairValueAdditions
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
   
Amortization (423)us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
   
Impairment reversal 0cbsh_ImpairmentAndImpairmentReversalsOfIntangibleAssetsFiniteLived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
   
Intangible Assets, Net (Including Goodwill) 6,149us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
  6,572us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CoreDepositsMember
Mortgage Servicing Rights [Member]      
Goodwill and Intangible Assets [Roll Forward]      
Originations 151us-gaap_ServicingAssetAtFairValueAdditions
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
   
Amortization (50)us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
   
Impairment reversal 15cbsh_ImpairmentAndImpairmentReversalsOfIntangibleAssetsFiniteLived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
   
Intangible Assets, Net (Including Goodwill) $ 994us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember
  $ 878us-gaap_IntangibleAssetsNetIncludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= cbsh_MortgageServicingRightsMember