Condensed Financial Statements of Parent Company - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related party receivables:
Total Assets	$ 1,055,895	$ 989,068
Liabilities
Dividends payable	1,053	1,007
Total liabilities	930,318	868,971
Stockholders' Equity	125,577	120,097	$ 114,493	$ 108,617
Total Liabilities and Stockholders' Equity	1,055,895	989,068
Farmers & Merchants State Bank [Member]
Assets
Cash	1,435	787
Related party receivables:
Dividends & Accounts receivable from subsidiaries	1,842	1,730
Accrued interest receivable Municipals	63	63
Securities - Municipals	17,290	16,388
Investment in subsidiaries	106,044	102,178
Total Assets	126,674	121,146
Liabilities
Accrued expenses	44	42
Dividends payable	1,053	1,007
Total liabilities	1,097	1,049
Stockholders' Equity	125,577	120,097
Total Liabilities and Stockholders' Equity	$ 126,674	$ 121,146