Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 28, 2020
Document And Entity Information [Abstract]
Document Quarterly Report	true
Title of 12(b) Security	Common Stock, $5 par value
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	38-1998421
Document Type	10-Q
Document Period End Date	Sep. 30, 2020
Amendment Flag	false
Entity Registrant Name	Comerica Incorporated
Entity Central Index Key	0000028412
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		139,087,862
Entity File Number	1-10706
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Address, Address Line One	1717 Main Street, MC 6404
Entity Address, City or Town	Dallas
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	75201
City Area Code	(214)
Local Phone Number	462-6831
Document Transition Report	false
Trading Symbol	CMA
Security Exchange Name	NYSE