Document and Entity Information Document (USD $)	12 Months Ended
	Dec. 31, 2011	Feb. 17, 2012	Jun. 30, 2010
Document and Entity Information [Abstract]
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-K
Document Period End Date	Dec. 31, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		109,215,443
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,759,873,138