Document and Entity Information Document (USD $)	6 Months Ended
	Jun. 30, 2013	Jul. 31, 2013	Jun. 30, 2012
Entity Information [Line Items]
Entity Registrant Name	FIRSTMERIT CORP /OH/
Entity Central Index Key	0000354869
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2013
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		165,043,294
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,810,181,825