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Securities - Gross Realized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Available-for-sale Securities, Gross Realized Gain (Loss), Disclosures [Abstract] |  |
Gross realized gains | $ 109 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | $ 122 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized losses | 0 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | 0 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Net realized gains | 109 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet | 122 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
Tax expense related to net realized gains | $ 37 fmao_AvailableForSaleSecuritiesGrossRealizedGainLossTaxExpenses | $ 42 fmao_AvailableForSaleSecuritiesGrossRealizedGainLossTaxExpenses

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- Definition Available for sale securities gross realized gain loss tax expenses. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration