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Employee Benefit Plans (Assumptions On A Weighted Average Basis, Used In Accounting For Plans) (Details)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]      
Determination of benefit obligation at year end, Discount rate 3.95%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.55%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 3.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Determination of benefit obligation at year end, Assumed credit on cash balance accounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationAssumedCreditOnCashBalanceAccounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationAssumedCreditOnCashBalanceAccounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationAssumedCreditOnCashBalanceAccounts
Determination of net periodic benefit cost for year ended, Discount rate 4.55%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 3.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Determination of net periodic benefit cost for year ended, Long-term rate of return on assets 6.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets 6.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Determination of net periodic benefit cost for year ended, Assumed credit on cash balance accounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostAssumedCreditOnCashBalanceAccounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostAssumedCreditOnCashBalanceAccounts 5.00%cbsh_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostAssumedCreditOnCashBalanceAccounts