UNAUDITED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Shares [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss)/Income [Member]	Total	Cumulative Effect, Period of Adoption, Adjustment [Member] Retained Earnings [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member]
Balance at Dec. 31, 2022	$ 8	$ 57,206	$ 449,932	$ (21,838)	$ 485,308
Balance (in shares) at Dec. 31, 2022	768,337
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	0	23,547	0	23,547
Other comprehensive income (loss), net of tax	0	0	0	5,638	5,638
Repurchase of common stock	$ 0	(5,591)	0	0	(5,591)
Repurchase of common stock (in shares)	(5,406)
Balance at Mar. 31, 2023	$ 8	51,615	473,479	(16,200)	508,902
Balance (in shares) at Mar. 31, 2023	762,931
Balance at Dec. 31, 2023	$ 7	36,852	525,360	(12,464)	$ 549,755	$ 40	$ 40
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible Enumeration]						fmcb:AccountingStandardsUpdate202302Member	fmcb:AccountingStandardsUpdate202302Member
Balance (in shares) at Dec. 31, 2023	747,971				747,971
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0	0	22,723	0	$ 22,723
Other comprehensive income (loss), net of tax	0	0	0	(1,850)	(1,850)
Repurchase of common stock	$ 0	(5,451)	0	0	(5,451)
Repurchase of common stock (in shares)	(5,201)
Balance at Mar. 31, 2024	$ 7	$ 31,401	$ 548,123	$ (14,314)	$ 565,217
Balance (in shares) at Mar. 31, 2024	742,770				742,770