Fair Market Value of Financial Assets and Liabilities (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains, noninterest income	$ (69,000)	$ 207,000
Total gains, noninterest expense	0	0
Recurring [Member] | Marketable Equity Securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	211,000	211,000
Total recognized gains (losses) Included in net income	0	0
Issues	0	0
Settlements	0	0
Ending balance	0	211,000
Total gains for the period included in net income	0	0
Recurring [Member] | Mortgage Servicing Rights [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2,364,000	2,282,000	2,282,000
Total recognized gains (losses) Included in net income	117,000	341,000
Issues	357,000	151,000
Settlements	(186,000)	(134,000)
Ending balance	2,652,000	2,640,000
Total gains for the period included in net income	117,000	341,000
Recurring [Member] | Fair Value [Member]
Available-for-sale securities [Abstract]
U.S. Treasury and government agencies	61,263,000		60,915,000
State and political subdivisions	120,448,000		113,221,000
U.S. government sponsored agency mortgage-backed securities	440,470,000		383,581,000
Marketable equity securities	55,329,000		45,626,000
Mortgage servicing rights	2,652,000		2,364,000
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available-for-sale securities [Abstract]
U.S. Treasury and government agencies	0		0
State and political subdivisions	0		0
U.S. government sponsored agency mortgage-backed securities	0		0
Marketable equity securities	55,329,000		45,626,000
Mortgage servicing rights	0		0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Available-for-sale securities [Abstract]
U.S. Treasury and government agencies	61,263,000		60,915,000
State and political subdivisions	120,448,000		113,221,000
U.S. government sponsored agency mortgage-backed securities	440,470,000		383,581,000
Marketable equity securities	0		0
Mortgage servicing rights	0		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Available-for-sale securities [Abstract]
U.S. Treasury and government agencies	0		0
State and political subdivisions	0		0
U.S. government sponsored agency mortgage-backed securities	0		0
Marketable equity securities	0		0
Mortgage servicing rights	2,652,000		2,364,000
Nonrecurring [Member]
Impaired loan (collateral dependent) [Abstract]
Impaired loans, fair value adjustments	500,000	100,000	500,000
Other real estate owned [Abstract]
Other real estate /assets owned, fair value adjustment	1,100,000	200,000	1,800,000
Nonrecurring [Member] | Fair Value [Member]
Assets measured-nonrecurring basis [Abstract]
Impaired loans (collateral dependent)	2,326,000		5,465,000
Other real estate /assets owned	4,365,000		13,892,000
Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets measured-nonrecurring basis [Abstract]
Impaired loans (collateral dependent)	0		0
Other real estate /assets owned	0		0
Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets measured-nonrecurring basis [Abstract]
Impaired loans (collateral dependent)	0		0
Other real estate /assets owned	0		0
Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets measured-nonrecurring basis [Abstract]
Impaired loans (collateral dependent)	2,326,000		5,465,000
Other real estate /assets owned	$ 4,365,000		$ 13,892,000