Derivative And Credit-Related Financial Instruments (Summary Of Internally Classified Watch List Letters Of Credit) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Total criticized standby and commercial letters of credit	$ 50	$ 44
As a percentage of total outstanding standby and commercial letters of credit	1.50%	1.30%

Total internally classified watch list standby and commercial letters of credit (generally consistent with regulatory defined special mention, substandard and doubtful).

No definition available.

Name:	cma_TotalInternallyClassifiedWatchListLettersOfCredit
Namespace Prefix:	cma_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant