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Investment Securities (Investment Securities, At Fair Value) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities $ 9,523,560us-gaap_AvailableForSaleSecurities $ 8,915,680us-gaap_AvailableForSaleSecurities
Trading securities 15,357us-gaap_TradingSecurities 19,993us-gaap_TradingSecurities
Non-marketable securities 106,875cbsh_NonMarketableSecurities 107,324cbsh_NonMarketableSecurities
Total investment securities 9,645,792cbsh_TotalInvestmentSecurities 9,042,997cbsh_TotalInvestmentSecurities
U.S. government and federal agency obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 501,407us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 505,696us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Government-sponsored enterprise obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 963,127us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 741,766us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State and municipal obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 1,813,201us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 1,619,171us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Agency mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 2,593,708us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbsh_AgencyMortgageBackedSecuritiesMember		 2,772,338us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbsh_AgencyMortgageBackedSecuritiesMember
Non-agency mortgage-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 382,744us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbsh_NonAgencyMortgageBackedSecuritiesMember		 246,983us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cbsh_NonAgencyMortgageBackedSecuritiesMember
Other asset-backed securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 3,091,993us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 2,844,071us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Other debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities 139,161us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 141,757us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Equity securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities $ 38,219us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 $ 43,898us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember