Securities (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2012
Available for sale
Amortized Cost	$ 896,609,000		$ 896,609,000		$ 978,477,000
Gross Unrealized Gains			13,973,000		23,289,000
Gross Unrealized Losses			(3,160,000)		(269,000)
Fair Value	907,422,000		907,422,000		1,001,497,000
Held to maturity
Amortized Cost	838,000		838,000
Gross Unrealized Gains	6,000
Fair Value	844,000		844,000
Available for sale, Amortized Cost
Due in one year or less	151,701,000		151,701,000
Due after one year through five years	478,362,000		478,362,000
Due after five years through ten years	182,805,000		182,805,000
Due after ten years	79,091,000		79,091,000
Amortized Cost	891,959,000		891,959,000
Available for sale, Fair Value
Due in one year or less	152,776,000		152,776,000
Due after one year through five years	481,436,000		481,436,000
Due after five years through ten years	185,574,000		185,574,000
Due after ten years	80,831,000		80,831,000
Fair Value	900,617,000		900,617,000
Held to maturity, Amortized Cost
Due after one year through five years	234,000		234,000
Due after five years through ten years	496,000		496,000
Due after ten years	108,000		108,000
Amortized Cost	838,000		838,000
Held to maturity, Fair Value
Due after one year through five years	235,000		235,000
Due after five years through ten years	499,000		499,000
Due after ten years	110,000		110,000
Fair Value	844,000		844,000
Realized gains and losses related to sales of securities available for sale
Gross security gains	82,000	511,000	82,000	576,000
Gross security (losses)				(1,000)
Net security gains	82,000	511,000	82,000	575,000
Tax provision related to net realized gains (losses)		200,000		200,000
Carrying amount of investment securities pledged as collateral	428,700,000		428,700,000		489,100,000
Available for sale, Fair Value
Less than 12 months	203,251,000		203,251,000		63,320,000
Greater than 12 months	4,582,000		4,582,000		3,311,000
Total	207,833,000		207,833,000		66,631,000
Available for sale, Unrealized Losses
Less than 12 months	3,096,000		3,096,000		220,000
Greater than 12 months	64,000		64,000		49,000
Total	3,160,000		3,160,000		269,000
Held to maturity, Fair Value
Less than 12 months	183,000		183,000
Total	183,000		183,000
Held to maturity, Unrealized Losses
Number of securities in unrealized loss position	312		312
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	1.50%		1.50%
Debt securities
Available for sale
Amortized Cost	891,959,000		891,959,000		976,026,000
Gross Unrealized Gains			11,818,000		21,810,000
Gross Unrealized Losses			(3,160,000)		(269,000)
Fair Value	900,617,000		900,617,000		997,567,000
U.S. Treasury securities
Available for sale
Amortized Cost	103,024,000		103,024,000		103,353,000
Gross Unrealized Gains			385,000		1,303,000
Gross Unrealized Losses			(71,000)
Fair Value	103,338,000		103,338,000		104,656,000
Available for sale, Fair Value
Less than 12 months	36,520,000		36,520,000
Total	36,520,000		36,520,000
Available for sale, Unrealized Losses
Less than 12 months	71,000		71,000
Total	71,000		71,000
Obligations of U.S. government corporations and agencies
Available for sale
Amortized Cost	285,452,000		285,452,000		363,583,000
Gross Unrealized Gains			3,258,000		6,616,000
Gross Unrealized Losses			(341,000)		(5,000)
Fair Value	288,369,000		288,369,000		370,194,000
Available for sale, Fair Value
Less than 12 months	25,264,000		25,264,000		10,155,000
Total	25,264,000		25,264,000		10,155,000
Available for sale, Unrealized Losses
Less than 12 months	341,000		341,000		5,000
Total	341,000		341,000		5,000
Obligations of states and political subdivisions
Available for sale
Amortized Cost	292,492,000		292,492,000		274,350,000
Gross Unrealized Gains			3,258,000		6,176,000
Gross Unrealized Losses			(2,463,000)		(238,000)
Fair Value	293,287,000		293,287,000		280,288,000
Held to maturity
Amortized Cost	838,000		838,000
Gross Unrealized Gains			6,000
Fair Value	844,000		844,000
Held to maturity, Amortized Cost
Amortized Cost	838,000		838,000
Held to maturity, Fair Value
Fair Value	844,000		844,000
Available for sale, Fair Value
Less than 12 months	116,930,000		116,930,000		37,958,000
Greater than 12 months	4,582,000		4,582,000		3,311,000
Total	121,512,000		121,512,000		41,269,000
Available for sale, Unrealized Losses
Less than 12 months	2,399,000		2,399,000		189,000
Greater than 12 months	64,000		64,000		49,000
Total	2,463,000		2,463,000		238,000
Held to maturity, Fair Value
Less than 12 months	183,000		183,000
Total	183,000		183,000
Obligations of states and political subdivisions | Maximum
Held to maturity, Unrealized Losses
Total			1,000
Residential mortgage-backed securities
Available for sale
Amortized Cost	185,584,000		185,584,000		210,139,000
Gross Unrealized Gains			4,802,000		7,576,000
Gross Unrealized Losses			(253,000)
Fair Value	190,133,000		190,133,000		217,715,000
Available for sale, Fair Value
Less than 12 months	15,183,000		15,183,000
Total	15,183,000		15,183,000
Available for sale, Unrealized Losses
Less than 12 months	253,000		253,000
Total	253,000		253,000
Corporate debt securities
Available for sale
Amortized Cost	25,407,000		25,407,000		24,601,000
Gross Unrealized Gains			115,000		139,000
Gross Unrealized Losses			(32,000)		(26,000)
Fair Value	25,490,000		25,490,000		24,714,000
Available for sale, Fair Value
Less than 12 months	9,354,000		9,354,000		15,207,000
Total	9,354,000		9,354,000		15,207,000
Available for sale, Unrealized Losses
Less than 12 months	32,000		32,000		26,000
Total	32,000		32,000		26,000
Mutual funds and other equity securities
Available for sale
Amortized Cost	4,650,000		4,650,000		2,451,000
Gross Unrealized Gains			2,155,000		1,479,000
Fair Value	$ 6,805,000		$ 6,805,000		$ 3,930,000