Nature of Operations and Summary of Significant Accounting Policies (Details)	12 Months Ended
Dec. 31, 2012 key_element
Accounting Policies [Abstract]
Number of key element used in assessing the appropriateness of the allowance for loan losses	3

Allowance for Loan Losses, Number Of Key Elements Applied to Determination Of Valuation Allowance

No definition available.

Name:	frme_AllowanceForLoanLossesNumberOfKeyElementsAppliedToDeterminationOfValuationAllowance
Namespace Prefix:	frme_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration