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Loans And Allowance For Loan Losses (Allowance For Loan Losses And Related Loan Balance Disaggregated On The Basis Of Impairment Methodology) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Loans And Allowance For Loan Losses [Line Items]    
Loans And Leases Receivable Excluding Impaired Financing Receivables Recorded Investment $ 11,643,151cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment $ 11,388,150cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
Impaired Financing Receivable, Recorded Investment 78,809us-gaap_ImpairedFinancingReceivableRecordedInvestment 81,088us-gaap_ImpairedFinancingReceivableRecordedInvestment
Loans and Leases Receivable Allowance Excluding Impaired Financing Receivable Related Allowance 148,224cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance 149,691cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
Impaired Financing Receivable, Related Allowance 5,308us-gaap_ImpairedFinancingReceivableRelatedAllowance 6,841us-gaap_ImpairedFinancingReceivableRelatedAllowance
Commercial Portfolio Segment [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Loans And Leases Receivable Excluding Impaired Financing Receivables Recorded Investment 6,837,826cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember		 6,606,123cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember
Impaired Financing Receivable, Recorded Investment 53,067us-gaap_ImpairedFinancingReceivableRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember		 55,551us-gaap_ImpairedFinancingReceivableRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember
Loans and Leases Receivable Allowance Excluding Impaired Financing Receivable Related Allowance 85,516cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember		 85,095cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember
Impaired Financing Receivable, Related Allowance 3,390us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember		 4,527us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= us-gaap_CommercialPortfolioSegmentMember
Personal Banking Portfolio Segment [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Loans And Leases Receivable Excluding Impaired Financing Receivables Recorded Investment 4,805,325cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 4,782,027cbsh_LoansAndLeasesReceivableExcludingImpairedFinancingReceivablesRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember
Impaired Financing Receivable, Recorded Investment 25,742us-gaap_ImpairedFinancingReceivableRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 25,537us-gaap_ImpairedFinancingReceivableRecordedInvestment
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember
Loans and Leases Receivable Allowance Excluding Impaired Financing Receivable Related Allowance 62,708cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 64,596cbsh_LoansAndLeasesReceivableAllowanceExcludingImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember
Impaired Financing Receivable, Related Allowance $ 1,918us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 $ 2,314us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ cbsh_FinancingReceivableAndAllowanceInformationByPortfolioSegmentAxis
= cbsh_PersonalBankingPortfolioSegmentMember