Loan Servicing (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Banking and Thrift [Abstract]
Unpaid balance of loans serviced for others	$ 514,294	$ 533,386	$ 549,618

Unpaid Principal Balance Of Loans Serviced For Others

No definition available.

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