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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 | $ 81,000 us-gaap_AmortizationOfIntangibleAssets | $ 120,000 us-gaap_AmortizationOfIntangibleAssets | $ 480,000 us-gaap_AmortizationOfIntangibleAssets |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Amortization expense |  |  |  | 323,000 us-gaap_AmortizationOfIntangibleAssets / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |
Custar [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  |  | 1,170,000 fmao_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 |  |  |  |  |  | $ 1,090,000 fmao_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmao_HicksvilleMember
Number of years amortized over an estimated remaining economic useful life of deposits | 7 years |  |  |  |  |

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