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Loans And Allowance For Loan Losses (Interest Income Recognized on Impaired Loans) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized $ 513us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod $ 702us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Business [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 135us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_BusinessLoanMember		 163us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_BusinessLoanMember
Real Estate - Construction And Land [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 80us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_ConstructionAndLandLoansMember		 161us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_ConstructionAndLandLoansMember
Real Estate - Business [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 15us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_BusinessRealEstateLoanMember		 30us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_BusinessRealEstateLoanMember
Real Estate - Personal [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 53us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_PersonalRealEstateLoanMember		 63us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= cbsh_PersonalRealEstateLoanMember
Consumer [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 52us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember		 75us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
Revolving Home Equity [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized 4us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_RevolvingCreditFacilityMember		 8us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_RevolvingCreditFacilityMember
Consumer Credit Card [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Interest income recognized $ 174us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember		 $ 202us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerCreditCardFinancingReceivableMember