Condensed Financial Statements of Parent Company - Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities
Net income	$ 20,095	$ 18,402	$ 14,949
Adjustments to reconcile net income to net cash provided by operating activities:
Other assets and liabilities	(573)	5,951	(7,466)
Net cash provided by operating activities	27,382	23,931	10,737
Activity in available-for-sale securities:
Maturities, prepayments and calls	91,003	67,071	29,691
Sales	11,843	11,100	10,081
Purchases	(183,563)	(110,166)	(13,866)
Net cash provided by (used in) investing activities	(177,724)	(149,316)	1,471
Cash Flows from Financing Activities
Payment of dividends	(7,186)	(6,345)	(4,956)
Purchase of treasury stock	(383)	(381)	(490)
Net cash provided by (used in) financing activities	274,752	138,316	(8,310)
Net Increase in Cash and Cash Equivalents	124,410	12,931	3,898
Cash and Cash Equivalents - Beginning of Year	51,296	38,365	34,467
Cash and Cash Equivalents - End of Year	175,706	51,296	38,365
Farmers & Merchants State Bank [Member]
Cash Flows from Operating Activities
Net income	20,095	18,402	14,949
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income	(11,652)	(5,724)	(6,887)
Net amortization of securities	154	99	190
Other assets and liabilities	2,814	(1,604)	(6,991)
Net cash provided by operating activities	11,411	11,173	1,261
Activity in available-for-sale securities:
Maturities, prepayments and calls	385	7,060	5,030
Sales		500	3,300
Purchases	(745)	(11,453)	(2,695)
Net cash provided by (used in) investing activities	(360)	(3,893)	5,635
Cash Flows from Financing Activities
Payment of dividends	(7,186)	(6,345)	(4,956)
Purchase of treasury stock	(383)	(381)	(490)
Net cash provided by (used in) financing activities	(7,569)	(6,726)	(5,446)
Net Increase in Cash and Cash Equivalents	3,482	554	1,450
Cash and Cash Equivalents - Beginning of Year	2,281	1,727	277
Cash and Cash Equivalents - End of Year	$ 5,763	$ 2,281	$ 1,727