Condensed Financial Statements of Parent Company - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related party receivables:
Total Assets	$ 1,909,544	$ 1,607,330
Liabilities
Dividends payable	1,889	1,768
Total liabilities	1,660,384	1,377,072
Stockholders' Equity	249,160	230,258	$ 143,287	$ 134,107
Total Liabilities and Stockholders' Equity	1,909,544	1,607,330
Farmers & Merchants State Bank [Member]
Assets
Cash	5,763	2,281
Related party receivables:
Dividends and accounts receivable from subsidiary	3,380	4,080
Accrued interest receivable - municipals	91	75
Securities - municipals	17,885	17,196
Certificate of deposits	1,727	1,974
Investment in subsidiaries	222,423	206,553
Total Assets	251,269	232,159
Liabilities
Accrued expenses	220	133
Dividends payable	1,889	1,768
Total liabilities	2,109	1,901
Stockholders' Equity	249,160	230,258
Total Liabilities and Stockholders' Equity	$ 251,269	$ 232,159