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Investment Securities Investment Securities (Credit Ratings In State And Municipal Bond Portfolio) (Details)
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]  
Percentage of State and Municipal Bond Portfolio, by credit rating 100.00%cbsh_PercentageOfStateAndMuniBondPortfolioCreditRating
Moody's, Aaa Rating [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Percentage of State and Municipal Bond Portfolio, by credit rating 9.30%cbsh_PercentageOfStateAndMuniBondPortfolioCreditRating
/ us-gaap_CreditRatingMoodysAxis
= us-gaap_MoodysAaaRatingMember
Moody's Aa Rating [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Percentage of State and Municipal Bond Portfolio, by credit rating 74.60%cbsh_PercentageOfStateAndMuniBondPortfolioCreditRating
/ us-gaap_CreditRatingMoodysAxis
= cbsh_MoodysAaRatingMember
Moody's A Rating [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Percentage of State and Municipal Bond Portfolio, by credit rating 15.40%cbsh_PercentageOfStateAndMuniBondPortfolioCreditRating
/ us-gaap_CreditRatingMoodysAxis
= cbsh_MoodysRatingMember
Moody's Not Rated [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Percentage of State and Municipal Bond Portfolio, by credit rating 0.70%cbsh_PercentageOfStateAndMuniBondPortfolioCreditRating
/ us-gaap_CreditRatingMoodysAxis
= cbsh_MoodysNoRatingMember