Securities (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Available-for-Sale [Abstract]
Amortized Cost	$ 660,611,000	$ 583,858,000
Gross Unrealized Gains	17,982,000	19,970,000
Gross Unrealized Losses	(1,083,000)	(485,000)
Fair Value	677,510,000	603,343,000
Held-to-Maturity [Abstract]
Amortized Cost	1,662,000	1,662,000
Gross Unrealized Gains	1,000	1,000
Gross Unrealized Losses	(7,000)	(4,000)
Fair Value	1,656,000	1,659,000
Available-for-Sale, Amortized Cost [Abstract]
Due in one year or less	7,602,000
Due after one through five years	23,004,000
Due after five through ten years	107,594,000
Due after ten years	38,624,000
U.S. government sponsored agency mortgage-backed securities	428,787,000
Total debt securities	605,611,000
Marketable equity securities	55,000,000
Total securities	660,611,000
Available-for-Sale, Fair Value [Abstract]
Due in one year or less	7,653,000
Due after one through five years	24,051,000
Due after five through ten years	109,952,000
Due after ten years	40,055,000
U.S. government sponsored agency mortgage-backed securities	440,470,000
Total debt securities	622,181,000
Marketable equity securities	55,329,000
Total securities	677,510,000
Held-to-Maturity, Amortized Cost [Abstract]
Due in one year or less	0
Due after one through five years	0
Due after five through ten years	1,182,000
Due after ten years	480,000
U.S. government sponsored agency mortgage-backed securities	0
Total debt securities	1,662,000
Marketable equity securities	0
Total securities	1,662,000
Held-to-Maturity, Fair Value [Abstract]
Due in one year or less	0
Due after one through five years	0
Due after five through ten years	1,183,000
Due after ten years	473,000
U.S. government sponsored agency mortgage-backed securities	0
Total debt securities	1,656,000
Marketable equity securities	0
Total securities	1,656,000
Securities pledged as collateral to secure public deposit and for other purposes	258,600,000	262,400,000
Amortized cost of securities sold under agreements to repurchase	237,000,000	237,300,000
Percentage of total investment with unrealized losses (in hundredths)	22.20%	14.80%
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	150,579,000	89,265,000
Total	151,685,000	89,265,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(1,070,000)	(485,000)
Total	(1,083,000)	(485,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	149,509,000	88,780,000
Total	150,602,000	88,780,000
Held-to-Maturity, Amortized Cost [Abstract]
Less Than 12 Months	480,000	480,000
Held-to-Maturity, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(7,000)	(4,000)
Held-to-Maturity, Fair Value [Abstract]
Less Than 12 Months	473,000	476,000
U.S. Treasury and government agencies [Member]
Available-for-Sale [Abstract]
Amortized Cost	61,100,000	60,625,000
Gross Unrealized Gains	435,000	463,000
Gross Unrealized Losses	(272,000)	(173,000)
Fair Value	61,263,000	60,915,000
Held-to-Maturity [Abstract]
Amortized Cost	480,000	480,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7,000)	(4,000)
Fair Value	473,000	476,000
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	48,078,000	47,576,000
Total	48,078,000	47,576,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(272,000)	(173,000)
Total	(272,000)	(173,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	47,806,000	47,403,000
Total	47,806,000	47,403,000
Held-to-Maturity, Amortized Cost [Abstract]
Less Than 12 Months	480,000	480,000
Held-to-Maturity, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(7,000)	(4,000)
Held-to-Maturity, Fair Value [Abstract]
Less Than 12 Months	473,000	476,000
State and political subdivisions [Member]
Available-for-Sale [Abstract]
Amortized Cost	115,724,000	107,987,000
Gross Unrealized Gains	4,949,000	5,369,000
Gross Unrealized Losses	(225,000)	(135,000)
Fair Value	120,448,000	113,221,000
Held-to-Maturity [Abstract]
Amortized Cost	1,182,000	1,182,000
Gross Unrealized Gains	1,000	1,000
Gross Unrealized Losses	0	0
Fair Value	1,183,000	1,183,000
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	17,012,000	11,126,000
12 Months or More	1,106,000
Total	18,118,000	11,126,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(212,000)	(135,000)
12 Months or More	(13,000)
Total	(225,000)	(135,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	16,800,000	10,991,000
12 Months or More	1,093,000
Total	17,893,000	10,991,000
U.S. government sponsored agency mortgage-backed securities [Member]
Available-for-Sale [Abstract]
Amortized Cost	428,787,000	370,246,000
Gross Unrealized Gains	11,992,000	13,347,000
Gross Unrealized Losses	(309,000)	(12,000)
Fair Value	440,470,000	383,581,000
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	55,489,000	10,563,000
Total	55,489,000	10,563,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(309,000)	(12,000)
Total	(309,000)	(12,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	55,180,000	10,551,000
Total	55,180,000	10,551,000
Debt securities [Member]
Available-for-Sale [Abstract]
Amortized Cost	605,611,000	538,858,000
Gross Unrealized Gains	17,376,000	19,179,000
Gross Unrealized Losses	(806,000)	(320,000)
Fair Value	622,181,000	557,717,000
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	120,579,000	69,265,000
12 Months or More	1,106,000
Total	121,685,000	69,265,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(793,000)	(320,000)
12 Months or More	(13,000)
Total	(806,000)	(320,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	119,786,000	68,945,000
12 Months or More	1,093,000
Total	120,879,000	68,945,000
Marketable equity securities [Member]
Available-for-Sale [Abstract]
Amortized Cost	55,000,000	45,000,000
Gross Unrealized Gains	606,000	791,000
Gross Unrealized Losses	(277,000)	(165,000)
Fair Value	55,329,000	45,626,000
Available-for-Sale, Amortized Cost [Abstract]
Less Than 12 Months	30,000,000	20,000,000
Total	30,000,000	20,000,000
Available-for-Sale, Gross Unrealized Losses [Abstract]
Less Than 12 Months	(277,000)	(165,000)
Total	(277,000)	(165,000)
Available-for-Sale, Fair Value [Abstract]
Less Than 12 Months	29,723,000	19,835,000
Total	$ 29,723,000	$ 19,835,000