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Asset Purchases - Additional Information (Detail) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2015
Dec. 13, 2013
Jul. 09, 2010
Business Acquisition [Line Items]            
Amortization expense $ 81,000us-gaap_AmortizationOfIntangibleAssets $ 120,000us-gaap_AmortizationOfIntangibleAssets $ 480,000us-gaap_AmortizationOfIntangibleAssets      
Scenario, Forecast [Member]            
Business Acquisition [Line Items]            
Amortization expense       323,000us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		    
Custar [Member]            
Business Acquisition [Line Items]            
Recognition of core deposit intangible asset with acquired purchase         1,170,000fmao_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase
/ us-gaap_BusinessAcquisitionAxis
= fmao_CustarMember		  
Number of years amortized over an estimated remaining economic useful life of deposits 7 years          
Hicksville [Member]            
Business Acquisition [Line Items]            
Recognition of core deposit intangible asset with acquired purchase           $ 1,090,000fmao_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase
/ us-gaap_BusinessAcquisitionAxis
= fmao_HicksvilleMember
Number of years amortized over an estimated remaining economic useful life of deposits 7 years