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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
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%

X

- 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

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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

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Namespace Prefix:	cma_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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