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Derivative And Credit-Related Financial Instruments (Summary Of Criticized Letters Of Credit) (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Total criticized standby and commercial letters of credit | $ 37 | $ 73
As a percentage of total outstanding standby and commercial letters of credit | 1.10% | 2.20%

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- Definition Total internally classified watch list standby and commercial letters of credit (generally consistent with regulatory defined special mention, substandard and doubtful). + References No definition available. + Details
Name: | cma_TotalInternallyClassifiedWatchListLettersOfCredit
Namespace Prefix: | cma_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total internally classified watch list standby and commercial letters of credit as a percentage of total standby and commercial letters of credit. + References No definition available. + Details
Name: | cma_WatchListLettersOfCreditAsPercentageOfTotalLettersOfCredit
Namespace Prefix: | cma_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration