Parent Company Financial Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Balance Sheets [Abstract]
Cash	$ 129,426	$ 101,660	$ 60,660
Investment Securities	486,383	542,912
Other Assets	73,038	73,575
Total Assets	1,974,686	1,919,684
Subordinated Debentures	10,310	10,310
Liabilities	1,769,653	1,730,338
Shareholders' Equity	205,033	189,346	173,241	164,727
Total Liabilities and Shareholders' Equity	1,974,686	1,919,684
Condensed Statements of Income [Abstract]
Interest Income	73,351	74,380	74,776
Tax Benefit	(13,985)	(12,642)	(12,169)
Net Income	23,349	22,209	21,118
Cash Flows from Operating Activities [Abstract]
Net Income	23,349	22,209	21,118
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities: [Abstract]
Net (Increase) Decrease in Other Assets	(434)	(2,750)	(6,360)
Net Cash Provided by Operating Activities	31,723	30,360	30,000
Investing Activities [Abstract]
Securities Purchased	(143,295)	(296,852)	(379,795)
Securities Sold or Matured	205,374	249,930	308,504
Net Cash Used by Investing Activities	(29,262)	(39,835)	(43,330)
Financing Activities [Abstract]
Stock Repurchases	(576)	0	(608)
Cash Dividends	(9,418)	(9,158)	(8,855)
Net Cash Used by Financing Activities	25,305	50,475	39,358
(Decrease) Increase in Cash and Cash Equivalents	27,766	41,000	26,028
Cash and Cash Equivalents at Beginning of Year	101,660	60,660	34,632
Cash and Cash Equivalents at End of Year	129,426	101,660	60,660
Parent Company [Member]
Condensed Balance Sheets [Abstract]
Cash	212	398	109
Investment in Farmers & Merchants Bank of Central California	214,755	199,177
Investment Securities	410	410
Capital Leases	0	1
Other Assets	267	49
Total Assets	215,644	200,035
Subordinated Debentures	10,310	10,310
Liabilities	301	379
Shareholders' Equity	205,033	189,346
Total Liabilities and Shareholders' Equity	215,644	200,035
Condensed Statements of Income [Abstract]
Equity in Undistributed Earnings in Farmers & Merchants Bank of Central California	13,247	12,715	11,938
Dividends from Subsidiary	10,900	10,325	10,000
Interest Income	10	10	10
Other Expenses, Net	(1,386)	(1,443)	(1,425)
Tax Benefit	578	602	595
Net Income	23,349	22,209	21,118
Cash Flows from Operating Activities [Abstract]
Net Income	23,349	22,209	21,118
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities: [Abstract]
Equity in Undistributed Net Earnings from Subsidiary	(13,247)	(12,715)	(11,938)
Net (Increase) Decrease in Other Assets	(216)	(3)	52
Net (Decrease) Increase in Liabilities	(78)	56	(1)
Net Cash Provided by Operating Activities	9,808	9,547	9,231
Investing Activities [Abstract]
Securities Purchased	0	(1,296)	0
Securities Sold or Matured	0	1,196	0
Net Cash Used by Investing Activities	0	(100)	0
Financing Activities [Abstract]
Stock Repurchases	(576)	0	(608)
Cash Dividends	(9,418)	(9,158)	(8,855)
Net Cash Used by Financing Activities	(9,994)	(9,158)	(9,463)
(Decrease) Increase in Cash and Cash Equivalents	(186)	289	(232)
Cash and Cash Equivalents at Beginning of Year	398	109	341
Cash and Cash Equivalents at End of Year	$ 212	$ 398	$ 109