Securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 item	Mar. 31, 2012	Dec. 31, 2012
Securities
Amortized Cost	$ 931,039,000		$ 978,477,000
Gross Unrealized Gains	21,786,000		23,289,000
Gross Unrealized Losses	(246,000)		(269,000)
Fair Value	952,579,000		1,001,497,000
Amortized Cost
Due in one year or less	143,583,000
Due after one year through five years	518,792,000
Due after five years through ten years	202,954,000
Due after ten years	62,147,000
Amortized Cost	927,476,000
Fair Value
Due in one year or less	144,951,000
Due after one year through five years	527,045,000
Due after five years through ten years	209,884,000
Due after ten years	65,364,000
Fair Value	947,244,000
Realized gains related to sales of securities	0	0
Realized losses related to sales of securities	0	0
Tax provision related to net realized gains and losses	0	0
Carrying amount of investment securities pledged as collateral	418,200,000		489,100,000
Fair Value
Less than 12 months	44,112,000		63,320,000
Greater than 12 months	4,282,000		3,311,000
Total	48,394,000		66,631,000
Unrealized Losses
Less than 12 months	190,000		220,000
Greater than 12 months	56,000		49,000
Total	246,000		269,000
Number of securities in unrealized loss position	74
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.51%
Debt securities
Securities
Amortized Cost	927,476,000		976,026,000
Gross Unrealized Gains	20,014,000		21,810,000
Gross Unrealized Losses	(246,000)		(269,000)
Fair Value	947,244,000		997,567,000
U.S. Treasury securities
Securities
Amortized Cost	103,225,000		103,353,000
Gross Unrealized Gains	1,292,000		1,303,000
Fair Value	104,517,000		104,656,000
Fair Value
Less than 12 months	355,000
Total	355,000
Obligations of U.S. government corporations and agencies
Securities
Amortized Cost	332,583,000		363,583,000
Gross Unrealized Gains	5,644,000		6,616,000
Gross Unrealized Losses	(16,000)		(5,000)
Fair Value	338,211,000		370,194,000
Fair Value
Less than 12 months	10,144,000		10,155,000
Total	10,144,000		10,155,000
Unrealized Losses
Less than 12 months	16,000		5,000
Total	16,000		5,000
Obligations of states and political subdivisions
Securities
Amortized Cost	276,195,000		274,350,000
Gross Unrealized Gains	5,819,000		6,176,000
Gross Unrealized Losses	(220,000)		(238,000)
Fair Value	281,794,000		280,288,000
Fair Value
Less than 12 months	26,454,000		37,958,000
Greater than 12 months	4,282,000		3,311,000
Total	30,736,000		41,269,000
Unrealized Losses
Less than 12 months	164,000		189,000
Greater than 12 months	56,000		49,000
Total	220,000		238,000
Residential mortgage-backed securities
Securities
Amortized Cost	190,989,000		210,139,000
Gross Unrealized Gains	7,105,000		7,576,000
Fair Value	198,094,000		217,715,000
Corporate debt securities
Securities
Amortized Cost	24,484,000		24,601,000
Gross Unrealized Gains	154,000		139,000
Gross Unrealized Losses	(10,000)		(26,000)
Fair Value	24,628,000		24,714,000
Fair Value
Less than 12 months	7,159,000		15,207,000
Total	7,159,000		15,207,000
Unrealized Losses
Less than 12 months	10,000		26,000
Total	10,000		26,000
Mutual funds and other equity securities
Securities
Amortized Cost	3,563,000		2,451,000
Gross Unrealized Gains	1,772,000		1,479,000
Fair Value	$ 5,335,000		$ 3,930,000