Basis of Accounting and Consolidation Basis of Accounting and Consolidation PA (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 branch	Dec. 31, 2014
Branch Purchase and Assumption Agreement [Abstract]
Number of Businesses Acquired	12us-gaap_NumberOfBusinessesAcquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		$ 160
Business Combination, Recognized Identifiable Assets and Liabilities Assumed, Loans		$ 502

Business Combination: Approximate Deposits to be acquired

No definition available.

Name:	fmbh_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedDeposits
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant