Derivative And Credit-Related Financial Instruments (Summary Of Internally Classified Watch List Letters Of Credit) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative And Credit-Related Financial Instruments [Abstract]
Total watch list standby and commercial letters of credit	$ 142	$ 144
As a percentage of total outstanding standby and commercial letters of credit	2.90%	2.90%

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