Purchase and Assumption - Assets and Liabilities of SCB Recorded at Provisional Fair Value (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Feb. 10, 2012
Business Combinations [Abstract]
Cash received from FDIC	$ 17.2

Business Acquisition Purchase Price Allocation Cash Received From Federal Deposit Insurance Corporation

No definition available.

Name:	frme_BusinessAcquisitionPurchasePriceAllocationCashReceivedFromFederalDepositInsuranceCorporation
Namespace Prefix:	frme_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant