XML 116 R99.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Earnings Per Share - Additional Information (Detail) | Jun. 04, 2021 USD ($) Director shares | Oct. 01, 2021 USD ($) shares | Nov. 20, 2020 shares | Jun. 05, 2020 USD ($) shares
--- | --- | --- | --- | ---
Earnings Per Share Basic [Line Items] |  |  |  |
Compensation annual basis committee award | $ | $ 6,000 |  |  | $ 4,000
Number of directors received shares | shares | 272 |  | 20 | 176
Number of directors received shares | Director | 10 |  |  |
Number of directors received prorated dollar value of shares | Director | 4 |  |  |
Perpetual Federal Savings Bank {Member] |  |  |  |
Earnings Per Share Basic [Line Items] |  |  |  |
Compensation annual basis committee award | $ |  | $ 1,523 |  |
Number of directors received shares | shares |  | 68 |  |

X
- Definition Compensation annual basis committee award. + References No definition available. + Details
Name: | fmao_CompensationAnnualBasisCommitteeAward
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of director received shares. + References No definition available. + Details
Name: | fmao_NumberOfDirectorReceivedShares
Namespace Prefix: | fmao_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of directors received prorated dollar value of shares. + References No definition available. + Details
Name: | fmao_NumberOfDirectorsReceivedProratedDollarValueOfShares
Namespace Prefix: | fmao_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of directors received shares. + References No definition available. + Details
Name: | fmao_NumberOfDirectorsReceivedShares
Namespace Prefix: | fmao_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_EarningsPerShareBasicLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fmao_PerpetualFederalSavingsBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |