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Balance Sheet Offsetting (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Swaps with Central Clearing Agency [Member]  
Swap Net Liability Position Fair Value $ 9.7cbsh_SwapNetLiabilityPositionFairValue
/ us-gaap_DerivativeByNatureAxis
= cbsh_SwapswithCentralClearingAgencyMember
Swaps with Central Clearing Agency [Member] | Investment Securities [Member]  
Value Of Collateral Posted 4.7cbsh_ValueOfCollateralPosted
/ us-gaap_DerivativeByNatureAxis
= cbsh_SwapswithCentralClearingAgencyMember
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= cbsh_InvestmentSecuritiesMember
Swaps with Central Clearing Agency [Member] | Cash [Member]  
Value Of Collateral Posted 7.5cbsh_ValueOfCollateralPosted
/ us-gaap_DerivativeByNatureAxis
= cbsh_SwapswithCentralClearingAgencyMember
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CashMember
Collateral Swap [Member]  
Collateral swap agreements 450.0cbsh_CollateralSwapAgreements
/ us-gaap_DerivativeByNatureAxis
= cbsh_CollateralSwapMember
Collateral Already Posted, Aggregate Fair Value 462.4us-gaap_CollateralAlreadyPostedAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= cbsh_CollateralSwapMember
Collateral Accepted, Aggregate Fair Value $ 493.7cbsh_CollateralAcceptedAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= cbsh_CollateralSwapMember