Condensed Financial Statements of Parent Company - Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows from Operating Activities
Net income	$ 3,194	$ 3,015	$ 2,974	$ 2,481	$ 2,772	$ 2,620	$ 2,597	$ 2,351	$ 11,664	$ 10,340	$ 9,646
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Gain on Sale of Available for Sale Securities									(588)	(451)	(494)
Changes in Assets and Liabilities:
Other Assets and Liabilities									(688)	1,275	971
Net Cash Provided by Operating Activities									12,430	14,809	19,163
Activity in available for sale securities:
Maturities, prepayments and calls									41,825	26,059	17,817
Sales									85,723	47,029	57,928
Purchases									(114,819)	(60,925)
Net cash provided by (used in) investing activities									(64,190)	(53,972)	22,507
Cash Flows from Financing Activities
Payment of dividends									(4,115)	(3,943)	(3,864)
Purchase of Treasury Stock									(194)	(803)	(576)
Net cash provided by (used) financing activities									58,064	36,886	(36,638)
Net Change in Cash and Cash Equivalents									6,304	(2,277)	5,032
Farmers & Merchants State Bank [Member]
Cash Flows from Operating Activities
Net income									11,664	10,340	9,646
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Equity in undistributed net income (Distributions in excess earnings) of subsidiaries									(5,835)	(4,266)	(4,087)
Accretion & Amortization of Securities									179	152	146
Gain on Sale of Available for Sale Securities										(124)
Changes in Assets and Liabilities:
Dividends receivable										475	(1,025)
Other Assets and Liabilities									355	156	(12)
Net Cash Provided by Operating Activities									6,363	6,733	4,668
Cash Flows from Investing Activities
Investing in Subsidiaries											(250)
Activity in available for sale securities:
Maturities, prepayments and calls									1,485	1,000
Sales										6,282
Purchases									(2,891)	(8,784)
Repayment of Subsidiary Subordinated Debt									0	0	0
Net cash provided by (used in) investing activities									(1,406)	(1,502)	(250)
Cash Flows from Financing Activities
Payment of dividends									(4,115)	(3,943)	(3,862)
Purchase of Treasury Stock									(194)	(803)	(576)
Net cash provided by (used) financing activities									(4,309)	(4,746)	(4,438)
Net Change in Cash and Cash Equivalents									648	485	(20)
Cash and Cash Equivalents
Cash and Cash Equivalents - Beginning of Year				$ 787				$ 302	787	302	322
Cash and Cash Equivalents - End of Year	$ 1,435				$ 787				$ 1,435	$ 787	$ 302