Parent Company Financial Information, Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Statements of Income [Abstract]
Interest Income	$ 28,076	$ 27,850	$ 26,531	$ 25,866	$ 24,449	$ 24,152	$ 23,363	$ 23,106	$ 108,323	$ 95,070	$ 86,750
Tax Benefit	(11,974)	(5,000)	(4,708)	(4,429)	(3,389)	(4,503)	(4,169)	(4,036)	(26,111)	(16,097)	(16,924)
Net Income	$ 3,781	$ 8,581	$ 8,187	$ 7,821	$ 7,681	$ 7,538	$ 7,332	$ 7,172	28,370	29,723	27,392
Parent Company [Member]
Condensed Statements of Income [Abstract]
Equity in Undistributed Earnings in Farmers & Merchants Bank of Central California									5,575	17,043	17,352
Dividends from Subsidiary									23,575	14,275	10,875
Interest Income									13	11	10
Other Expenses, Net									(1,552)	(2,485)	(1,451)
Tax Benefit									759	879	606
Net Income									$ 28,370	$ 29,723	$ 27,392