Condensed Financial Statements of Parent Company - Statements of Income and Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income
Interest - municipals / certificates of deposit									$ 1,079	$ 837	$ 1,095
Loss on sales of available-for-sale securities									270	(26)	(9)
Total interest income	$ 18,139	$ 17,293	$ 17,273	$ 17,464	$ 17,149	$ 17,012	$ 18,283	$ 15,862	70,169	68,306	46,429
Income Before Income Taxes and Equity in Undistributed Earnings of Subsidiaries	8,467	5,697	5,965	5,077	5,852	5,234	7,728	3,948
Income Taxes (Benefit)	1,691	1,287	1,161	972	1,129	959	1,548	724	5,111	4,360	3,301
Net Income	$ 6,776	$ 4,410	$ 4,804	$ 4,105	$ 4,723	$ 4,275	$ 6,180	$ 3,224	20,095	18,402	14,949
Other Comprehensive Income (Loss):
Comprehensive Income									24,694	22,514	14,120
Farmers & Merchants State Bank [Member]
Income
Dividends from subsidiaries									8,800	13,137	8,350
Interest - municipals / certificates of deposit									382	374	389
Loss on sales of available-for-sale securities											(19)
Total interest income									9,182	13,511	8,720
Operating Expenses									923	999	1,020
Income Before Income Taxes and Equity in Undistributed Earnings of Subsidiaries									8,259	12,512	7,700
Income Taxes (Benefit)									(184)	(166)	(362)
Income from operation before equity in undistributed earnings of subsidiary									8,443	12,678	8,062
Equity in undistributed earnings of subsidiaries									11,652	5,724	6,887
Net Income									20,095	18,402	14,949
Other Comprehensive Income (Loss):
Unrealized gains (losses) on securities									4,599	4,112	(829)
Comprehensive Income									$ 24,694	$ 22,514	$ 14,120