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Employee Benefit Plans (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net periodic defined benefit cost $ 40,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost $ 88,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost $ 81,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Estimated average life of pension plan in years 15cma_EstimatedAverageLifeOfFund    
Fair value of plan assets in defined benefit pension plan 2,531,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 2,028,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Estimated employer contributions for defined benefit plans in year 2015 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Employer core matching cash contribution as percent of employee contribution 100.00%cma_DefinedContributionPlanTotalQualifiedEarningsMatchingCashContributionsByEmployerPercent    
Percent of qualified earnings contributed by employee matched by the Corporation 4.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent    
Defined contribution plan expense 22,000,000us-gaap_DefinedContributionPlanCostRecognized 21,000,000us-gaap_DefinedContributionPlanCostRecognized 20,000,000us-gaap_DefinedContributionPlanCostRecognized
Profit sharing plan annual contribution to the individual account of each eligible employee, minimum percent of annual compensation 3.00%cma_AnnualContributionForProfitSharingPlanToIndividualAccountOfEachEligibleEmployeeMinimum    
Profit sharing plan annual contribution to the individual account of each eligible employee, maximum percent of annual compensation 8.00%cma_AnnualContributionForProfitSharingPlanToIndividualAccountOfEachEligibleEmployeeMaximum    
Profit sharing plan expense included in employee benefits expense 10,000,000cma_ProfitSharingPlanCostsRecognized 7,000,000cma_ProfitSharingPlanCostsRecognized 7,000,000cma_ProfitSharingPlanCostsRecognized
Equity securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Target allocation, minimum percent 36.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		    
Target allocation, maximum percent 56.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		    
Debt securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Target allocation, minimum percent 44.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember		    
Target allocation, maximum percent 64.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember		    
Defined Benefit Pension Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net periodic defined benefit cost 39,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 86,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 75,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Non-Qualified Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net periodic defined benefit cost 16,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cma_NonQualifiedPlansMember		 18,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cma_NonQualifiedPlansMember		 19,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cma_NonQualifiedPlansMember
Fair value of plan assets in defined benefit pension plan 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cma_NonQualifiedPlansMember		 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cma_NonQualifiedPlansMember		  
Postretirement Benefit Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net periodic defined benefit cost 1,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 2,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 6,000,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Fair value of plan assets in defined benefit pension plan $ 67,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 $ 67,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 $ 72,000,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember