Securities (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 item	Mar. 31, 2014	Dec. 31, 2014
Available for sale
Amortized Cost	$ 817,883,000		$ 749,364,000
Gross Unrealized Gains	14,017,000		10,740,000
Gross Unrealized Losses	(286,000)		(1,039,000)
Fair Value	831,614,000		759,065,000
Held to maturity
Amortized Cost	35,037,000		2,373,000
Gross Unrealized Gains	376,000		55,000
Gross Unrealized Losses	(26,000)		(3,000)
Fair Value	35,387,000		2,425,000
Available for sale, Amortized Cost
Due in one year or less	121,390,000
Due after one year through five years	371,768,000
Due after five years through ten years	134,541,000
Due after ten years	184,066,000
Amortized Cost	811,765,000
Available for sale, Fair Value
Due in one year or less	121,890,000
Due after one year through five years	375,370,000
Due after five years through ten years	138,857,000
Due after ten years	188,082,000
Fair Value	824,199,000
Held to maturity, Amortized Cost
Due in one year or less	1,075,000
Due after one year through five years	6,249,000
Due after five years through ten years	19,337,000
Due after ten years	8,376,000
Amortized Cost	35,037,000		2,373,000
Held to maturity, Fair Value
Due in one year or less	1,077,000
Due after one year through five years	6,286,000
Due after five years through ten years	19,585,000
Due after ten years	8,439,000
Fair Value	35,387,000		2,425,000
Realized gains and losses related to sales of securities available for sale
Gross security gains	1,000	57,000
Gross security (losses)		(14,000)
Net security gains	1,000	43,000
Carrying amount of investment securities pledged as collateral	556,600,000		536,200,000
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	28,814,000		67,415,000
Continuous unrealized losses existing greater than 12 months, gross	30,380,000		65,898,000
Total	59,194,000		133,313,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(93,000)		(234,000)
Continuous unrealized losses existing greater than 12 months, gross	(193,000)		(805,000)
Total	(286,000)		(1,039,000)
Held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	1,952,000		534,000
Total	1,952,000		534,000
Held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(26,000)		(3,000)
Total	(26,000)		(3,000)
Number of securities in unrealized loss position	91
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.50%
U.S. Treasury securities
Available for sale
Amortized Cost	65,182,000		50,280,000
Gross Unrealized Gains	748,000		328,000
Gross Unrealized Losses			(2,000)
Fair Value	65,930,000		50,606,000
Available for sale, Fair Value
Continuous unrealized losses existing greater than 12 months, gross			366,000
Total			366,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing greater than 12 months, gross			(2,000)
Total			(2,000)
Obligations of U.S. government corporations and agencies
Available for sale
Amortized Cost	156,404,000		166,207,000
Gross Unrealized Gains	994,000		981,000
Gross Unrealized Losses	(13,000)		(178,000)
Fair Value	157,385,000		167,010,000
Available for sale, Fair Value
Continuous unrealized losses existing greater than 12 months, gross	10,142,000		25,118,000
Total	10,142,000		25,118,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing greater than 12 months, gross	(13,000)		(178,000)
Total	(13,000)		(178,000)
Obligations of states and political subdivisions
Available for sale
Amortized Cost	206,112,000		218,250,000
Gross Unrealized Gains	3,194,000		2,672,000
Gross Unrealized Losses	(225,000)		(761,000)
Fair Value	209,081,000		220,161,000
Held to maturity
Amortized Cost	34,027,000		1,359,000
Gross Unrealized Gains	320,000		15,000
Gross Unrealized Losses	(26,000)		(3,000)
Fair Value	34,321,000		1,371,000
Held to maturity, Amortized Cost
Amortized Cost	34,027,000		1,359,000
Held to maturity, Fair Value
Fair Value	34,321,000		1,371,000
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	11,118,000		40,385,000
Continuous unrealized losses existing greater than 12 months, gross	20,238,000		40,201,000
Total	31,356,000		80,586,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(45,000)		(140,000)
Continuous unrealized losses existing greater than 12 months, gross	(180,000)		(621,000)
Total	(225,000)		(761,000)
Held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	1,952,000		534,000
Total	1,952,000		534,000
Held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(26,000)		(3,000)
Total	(26,000)		(3,000)
Residential mortgage-backed securities
Available for sale
Amortized Cost	274,279,000		230,596,000
Gross Unrealized Gains	6,648,000		5,062,000
Gross Unrealized Losses	(26,000)		(22,000)
Fair Value	280,901,000		235,636,000
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	9,484,000		10,630,000
Total	9,484,000		10,630,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(26,000)		(22,000)
Total	(26,000)		(22,000)
Corporate debt securities
Available for sale
Amortized Cost	109,788,000		79,087,000
Gross Unrealized Gains	1,136,000		296,000
Gross Unrealized Losses	(22,000)		(76,000)
Fair Value	110,902,000		79,307,000
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	8,212,000		16,400,000
Continuous unrealized losses existing greater than 12 months, gross			213,000
Total	8,212,000		16,613,000
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(22,000)		(72,000)
Continuous unrealized losses existing greater than 12 months, gross			(4,000)
Total	(22,000)		(76,000)
Debt securities
Available for sale
Amortized Cost	811,765,000		744,420,000
Gross Unrealized Gains	12,720,000		9,339,000
Gross Unrealized Losses	(286,000)		(1,039,000)
Fair Value	824,199,000		752,720,000
Mutual funds and other equity securities
Available for sale
Amortized Cost	6,118,000		4,944,000
Gross Unrealized Gains	1,297,000		1,401,000
Fair Value	7,415,000		6,345,000
Commercial mortgage-backed securities
Available for sale
Amortized Cost			1,014,000
Gross Unrealized Gains			40,000
Fair Value			1,054,000
Held to maturity
Amortized Cost	1,010,000
Gross Unrealized Gains	56,000
Fair Value	1,066,000
Held to maturity, Amortized Cost
Amortized Cost	1,010,000
Held to maturity, Fair Value
Fair Value	$ 1,066,000