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Investment Securities (Cash Flow Model Inputs Used To Calculate Credit Losses) (Details)
Dec. 31, 2014
Minimum [Member]  
Investment [Line Items]  
Prepayment CPR 2.00%cbsh_PrepaymentCprInputVariable
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Projected cumulative default 20.00%cbsh_ProjectedCumulativeDefaultInputVariable
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Credit support 0.00%cbsh_CreditSupportInputVariable
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Loss severity 23.00%cbsh_LossSeverityInputVariable
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]  
Investment [Line Items]  
Prepayment CPR 25.00%cbsh_PrepaymentCprInputVariable
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Projected cumulative default 58.00%cbsh_ProjectedCumulativeDefaultInputVariable
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Credit support 18.00%cbsh_CreditSupportInputVariable
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Loss severity 77.00%cbsh_LossSeverityInputVariable
/ us-gaap_RangeAxis
= us-gaap_MaximumMember