ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently adopted accounting pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Jan. 01, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total FMC stockholders’ equity	$ 246,256	$ 242,242		$ 240,864	$ 234,862	$ 228,024	$ 219,041
Allowance for credit losses: loans	23,175	22,044	$ 20,653	17,183	16,081	14,357	13,732
Allowance for credit losses on off-balance sheet exposures			3,900
Allowance for credit losses: held-to-maturity securities			71
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total FMC stockholders’ equity	$ 54,261	$ 51,143			$ 46,416	$ 40,195	$ 30,189
Cumulative change in accounting principle | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total FMC stockholders’ equity				$ 3,470
Cumulative change in accounting principle | Retained Earnings | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total FMC stockholders’ equity			5,300
Allowance for credit losses: loans			3,500
Allowance for credit losses on off-balance sheet exposures			3,500
Allowance for credit losses: held-to-maturity securities			$ 100