Securities (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2013 item	Dec. 31, 2012
Securities
Amortized Cost			$ 911,762,000	$ 978,477,000
Gross Unrealized Gains			13,572,000	23,289,000
Gross Unrealized Losses			(3,769,000)	(269,000)
Fair Value			921,565,000	1,001,497,000
Amortized Cost
Due in one year or less			160,492,000
Due after one year through five years			494,329,000
Due after five years through ten years			195,331,000
Due after ten years			56,581,000
Amortized Cost			906,733,000
Fair Value
Due in one year or less			161,813,000
Due after one year through five years			497,057,000
Due after five years through ten years			197,695,000
Due after ten years			57,994,000
Fair Value			914,559,000
Realized gains and losses related to sales of securities
Gross security gains	64,000	64,000
Net security gains	64,000	64,000
Carrying amount of investment securities pledged as collateral			471,500,000	489,100,000
Fair Value
Less than 12 months			238,754,000	63,320,000
Greater than 12 months			4,489,000	3,311,000
Total			243,243,000	66,631,000
Unrealized Losses
Less than 12 months			3,692,000	220,000
Greater than 12 months			77,000	49,000
Total			3,769,000	269,000
Number of securities in unrealized loss position			356
Securities in unrealized loss position as a percentage of aggregate carrying value of investments			1.50%
Debt securities
Securities
Amortized Cost			906,733,000	976,026,000
Gross Unrealized Gains			11,595,000	21,810,000
Gross Unrealized Losses			(3,769,000)	(269,000)
Fair Value			914,559,000	997,567,000
U.S. Treasury securities
Securities
Amortized Cost			103,097,000	103,353,000
Gross Unrealized Gains			301,000	1,303,000
Gross Unrealized Losses			(223,000)
Fair Value			103,175,000	104,656,000
Fair Value
Less than 12 months			51,050,000
Total			51,050,000
Unrealized Losses
Less than 12 months			223,000
Total			223,000
Obligations of U.S. government corporations and agencies
Securities
Amortized Cost			311,442,000	363,583,000
Gross Unrealized Gains			3,723,000	6,616,000
Gross Unrealized Losses			(525,000)	(5,000)
Fair Value			314,640,000	370,194,000
Fair Value
Less than 12 months			25,119,000	10,155,000
Total			25,119,000	10,155,000
Unrealized Losses
Less than 12 months			525,000	5,000
Total			525,000	5,000
Obligations of states and political subdivisions
Securities
Amortized Cost			294,299,000	274,350,000
Gross Unrealized Gains			3,462,000	6,176,000
Gross Unrealized Losses			(2,694,000)	(238,000)
Fair Value			295,067,000	280,288,000
Fair Value
Less than 12 months			129,570,000	37,958,000
Greater than 12 months			4,489,000	3,311,000
Total			134,059,000	41,269,000
Unrealized Losses
Less than 12 months			2,617,000	189,000
Greater than 12 months			77,000	49,000
Total			2,694,000	238,000
Residential mortgage-backed securities
Securities
Amortized Cost			173,456,000	210,139,000
Gross Unrealized Gains			4,022,000	7,576,000
Gross Unrealized Losses			(237,000)
Fair Value			177,241,000	217,715,000
Fair Value
Less than 12 months			12,829,000
Total			12,829,000
Unrealized Losses
Less than 12 months			237,000
Total			237,000
Corporate debt securities
Securities
Amortized Cost			24,439,000	24,601,000
Gross Unrealized Gains			87,000	139,000
Gross Unrealized Losses			(90,000)	(26,000)
Fair Value			24,436,000	24,714,000
Fair Value
Less than 12 months			20,186,000	15,207,000
Total			20,186,000	15,207,000
Unrealized Losses
Less than 12 months			90,000	26,000
Total			90,000	26,000
Mutual funds and other equity securities
Securities
Amortized Cost			5,029,000	2,451,000
Gross Unrealized Gains			1,977,000	1,479,000
Fair Value			$ 7,006,000	$ 3,930,000