Derivative Financial Instruments - Schedule of Effects of Interest Rate Swap Agreements on Consolidated Statements of Income (Detail) - Designated as Hedging Instrument [Member] - Interest Rate Swap [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Location, Statement of Income, Balance [Axis]: us-gaap:InterestAndDividendIncomeOperating
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Effects of derivatives on consolidated statement of income	$ (231)	$ 13	$ (633)	$ 18
Location, Statement of Income, Balance [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeases
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Effects of derivatives on consolidated statement of income	(242)	(12)	(686)	(57)
Location, Statement of Income, Balance [Axis]: fmao:InterestIncomeFromFederalFundsSoldAndOther
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Effects of derivatives on consolidated statement of income	$ 11	$ 25	$ 53	$ 75

Derivative Financial Instruments - Schedule of Effects of Interest Rate Swap Agreements on Consolidated Statements of Income (Detail) - Designated as Hedging Instrument [Member] - Interest Rate Swap [Member] - USD ($)
$ in Thousands

3 Months Ended

9 Months Ended

Sep. 30, 2026

Sep. 30, 2025

Sep. 30, 2026

Sep. 30, 2025

Location, Statement of Income, Balance [Axis]: us-gaap:InterestAndDividendIncomeOperating

Derivative Instruments and Hedging Activities Disclosures [Line Items]

Effects of derivatives on consolidated statement of income

$ (231)

$ 13

$ (633)