Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Securities
Amortized Cost	$ 978,477,000	$ 809,439,000
Gross Unrealized Gains	23,289,000	22,537,000
Gross Unrealized Losses	(269,000)	(227,000)
Fair Value	1,001,497,000	831,749,000
Amortized Cost
Due in one year or less	140,695,000
Due after one year through five years	548,425,000
Due after five years through ten years	217,700,000
Due after ten years	69,206,000
Amortized Cost	976,026,000
Fair Value
Due in one year or less	141,997,000
Due after one year through five years	557,505,000
Due after five years through ten years	225,070,000
Due after ten years	72,995,000
Fair Value	997,567,000
Realized gains and losses related to sales of securities
Gross security gains	1,602,000	174,000	1,025,000
Gross security (losses)	(5,000)	(4,000)	(7,000)
Net security gains	1,597,000	170,000	1,018,000
Tax provision related to net realized gains and losses	600,000		400,000
Carrying amount of investment securities pledged as collateral	489,100,000	359,900,000
Fair Value
Continuous unrealized losses existing for less than 12 months, gross	63,320,000	53,535,000
Continuous unrealized losses existing greater than 12 months, gross	3,311,000
Total, gross	66,631,000	53,535,000
Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	220,000	227,000
Continuous unrealized losses existing greater than 12 months, gross	49,000
Total, gross	269,000	227,000
Number of securities in unrealized loss position	110
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.40%
Debt securities
Securities
Amortized Cost	976,026,000	809,220,000
Gross Unrealized Gains	21,810,000	21,208,000
Gross Unrealized Losses	(269,000)	(227,000)
Fair Value	997,567,000	830,201,000
U.S. Treasury securities
Securities
Amortized Cost	103,353,000	45,550,000
Gross Unrealized Gains	1,303,000	485,000
Fair Value	104,656,000	46,035,000
Obligations of U.S. government corporations and agencies
Securities
Amortized Cost	363,583,000	339,983,000
Gross Unrealized Gains	6,616,000	9,083,000
Gross Unrealized Losses	(5,000)	(35,000)
Fair Value	370,194,000	349,031,000
Fair Value
Continuous unrealized losses existing for less than 12 months, gross	10,155,000	15,615,000
Total, gross	10,155,000	15,615,000
Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	5,000	35,000
Total, gross	5,000	35,000
Obligations of states and political subdivisions
Securities
Amortized Cost	274,350,000	149,368,000
Gross Unrealized Gains	6,176,000	5,193,000
Gross Unrealized Losses	(238,000)	(124,000)
Fair Value	280,288,000	154,437,000
Fair Value
Continuous unrealized losses existing for less than 12 months, gross	37,958,000	21,037,000
Continuous unrealized losses existing greater than 12 months, gross	3,311,000
Total, gross	41,269,000	21,037,000
Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	189,000	124,000
Continuous unrealized losses existing greater than 12 months, gross	49,000
Total, gross	238,000	124,000
Residential mortgage-backed securities
Securities
Amortized Cost	210,139,000	271,787,000
Gross Unrealized Gains	7,576,000	6,374,000
Gross Unrealized Losses		(46,000)
Fair Value	217,715,000	278,115,000
Fair Value
Continuous unrealized losses existing for less than 12 months, gross		16,428,000
Total, gross		16,428,000
Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross		46,000
Total, gross		46,000
Corporate debt securities
Securities
Amortized Cost	24,601,000	2,532,000
Gross Unrealized Gains	139,000	73,000
Gross Unrealized Losses	(26,000)	(22,000)
Fair Value	24,714,000	2,583,000
Fair Value
Continuous unrealized losses existing for less than 12 months, gross	15,207,000	455,000
Total, gross	15,207,000	455,000
Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	26,000	22,000
Total, gross	26,000	22,000
Mutual funds and other equity securities
Securities
Amortized Cost	2,451,000	219,000
Gross Unrealized Gains	1,479,000	1,329,000
Fair Value	$ 3,930,000	$ 1,548,000