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Loans And Allowance For Loan Losses (Total Average Impaired Loans) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Loans And Allowance For Loan Losses [Line Items]    
Non-accrual loans $ 45,246cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus $ 41,229cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
Restructured loans (accruing) 53,196cbsh_FinancingReceivableModificationsAverageRecordedInvestment 64,385cbsh_FinancingReceivableModificationsAverageRecordedInvestment
Average impaired loans 98,442us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment 105,614us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Commercial Portfolio Segment [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Non-accrual loans 38,114cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember		 35,900cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember
Restructured loans (accruing) 33,156cbsh_FinancingReceivableModificationsAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember		 40,251cbsh_FinancingReceivableModificationsAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember
Average impaired loans 71,270us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember		 76,151us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= us-gaap_CommercialPortfolioSegmentMember
Personal Banking Portfolio Segment [Member]    
Loans And Allowance For Loan Losses [Line Items]    
Non-accrual loans 7,132cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 5,329cbsh_FinancingReceivableAverageRecordedInvestmentNonaccrualStatus
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember
Restructured loans (accruing) 20,040cbsh_FinancingReceivableModificationsAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 24,134cbsh_FinancingReceivableModificationsAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember
Average impaired loans $ 27,172us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember		 $ 29,463us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ cbsh_AverageImpairedLoansAxis
= cbsh_PersonalBankingPortfolioSegmentMember