Securities (Details) (USD $)
12 Months Ended
Dec. 31, 2014
item
Dec. 31, 2013
Dec. 31, 2012
Available for sale      
Amortized Cost $ 749,364,000 $ 833,735,000  
Gross Unrealized Gains 10,740,000 11,350,000  
Gross Unrealized Losses (1,039,000) (3,775,000)  
Fair Value 759,065,000 841,310,000  
Held to maturity      
Amortized Cost 2,373,000 834,000  
Gross Unrealized Gains 55,000 1,000  
Gross Unrealized Losses (3,000) (4,000)  
Fair Value 2,425,000 831,000  
Available for sale, Amortized Cost      
Due in one year or less 125,672,000    
Due after one year through five years 347,781,000    
Due after five years through ten years 120,891,000    
Due after ten years 150,076,000    
Amortized Cost 744,420,000    
Available for sale, Fair Value      
Due in one year or less 126,252,000    
Due after one year through five years 349,168,000    
Due after five years through ten years 124,483,000    
Due after ten years 152,817,000    
Fair Value 752,720,000    
Held to maturity, Amortized Cost      
Due in one year or less 72,000    
Due after one year through five years 636,000    
Due after five years through ten years 1,665,000    
Amortized Cost 2,373,000 834,000  
Held to maturity, Fair Value      
Due in one year or less 72,000    
Due after one year through five years 636,000    
Due after five years through ten years 1,717,000    
Fair Value 2,425,000 831,000  
Realized gains and losses related to sales of securities available for sale      
Gross security gains 807,000 599,000 1,602,000
Gross security (losses) (31,000) (46,000) (5,000)
Net security gains 776,000 553,000 1,597,000
Tax provision related to net realized gains (losses) 300,000 200,000 600,000
Carrying amount of investment securities pledged as collateral 536,200,000 428,700,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 67,415,000 199,636,000  
Continuous unrealized losses existing greater than 12 months, gross 65,898,000 8,492,000  
Total 133,313,000 208,128,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 234,000 3,481,000  
Continuous unrealized losses existing greater than 12 months, gross 805,000 294,000  
Total 1,039,000 3,775,000  
Held to maturity, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 534,000 597,000  
Total 534,000 597,000  
Held to maturity, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 3,000 4,000  
Total 3,000 4,000  
Number of securities in unrealized loss position 202    
Securities in unrealized loss position as a percentage of aggregate carrying value of investments 0.80%    
Debt securities      
Available for sale      
Amortized Cost 744,420,000 829,621,000  
Gross Unrealized Gains 9,339,000 9,598,000  
Gross Unrealized Losses (1,039,000) (3,775,000)  
Fair Value 752,720,000 835,444,000  
U.S. Treasury securities      
Available for sale      
Amortized Cost 50,280,000 102,463,000  
Gross Unrealized Gains 328,000 244,000  
Gross Unrealized Losses (2,000) (67,000)  
Fair Value 50,606,000 102,640,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross   25,830,000  
Continuous unrealized losses existing greater than 12 months, gross 366,000    
Total 366,000 25,830,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross   67,000  
Continuous unrealized losses existing greater than 12 months, gross 2,000    
Total 2,000 67,000  
Obligations of U.S. government corporations and agencies      
Available for sale      
Amortized Cost 166,207,000 254,998,000  
Gross Unrealized Gains 981,000 2,741,000  
Gross Unrealized Losses (178,000) (328,000)  
Fair Value 167,010,000 257,411,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross   25,946,000  
Continuous unrealized losses existing greater than 12 months, gross 25,118,000    
Total 25,118,000 25,946,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross   328,000  
Continuous unrealized losses existing greater than 12 months, gross 178,000    
Total 178,000 328,000  
Obligations of states and political subdivisions      
Available for sale      
Amortized Cost 218,250,000 272,077,000  
Gross Unrealized Gains 2,672,000 2,887,000  
Gross Unrealized Losses (761,000) (2,812,000)  
Fair Value 220,161,000 272,152,000  
Held to maturity      
Amortized Cost 1,359,000 834,000  
Gross Unrealized Gains 15,000 1,000  
Gross Unrealized Losses (3,000) (4,000)  
Fair Value 1,371,000 831,000  
Held to maturity, Amortized Cost      
Amortized Cost 1,359,000 834,000  
Held to maturity, Fair Value      
Fair Value 1,371,000 831,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 40,385,000 92,703,000  
Continuous unrealized losses existing greater than 12 months, gross 40,201,000 8,492,000  
Total 80,586,000 101,195,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 140,000 2,518,000  
Continuous unrealized losses existing greater than 12 months, gross 621,000 294,000  
Total 761,000 2,812,000  
Held to maturity, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 534,000 597,000  
Total 534,000 597,000  
Held to maturity, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 3,000 4,000  
Total 3,000 4,000  
Residential mortgage-backed securities      
Available for sale      
Amortized Cost 230,596,000 174,699,000  
Gross Unrealized Gains 5,062,000 3,571,000  
Gross Unrealized Losses (22,000) (535,000)  
Fair Value 235,636,000 177,735,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 10,630,000 53,543,000  
Total 10,630,000 53,543,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 22,000 535,000  
Total 22,000 535,000  
Corporate debt securities      
Available for sale      
Amortized Cost 79,087,000 25,384,000  
Gross Unrealized Gains 296,000 155,000  
Gross Unrealized Losses (76,000) (33,000)  
Fair Value 79,307,000 25,506,000  
Available for sale, Fair Value      
Continuous unrealized losses existing for less than 12 months, gross 16,400,000 1,614,000  
Continuous unrealized losses existing greater than 12 months, gross 213,000    
Total 16,613,000 1,614,000  
Available for sale, Unrealized Losses      
Continuous unrealized losses existing for less than 12 months, gross 72,000 33,000  
Continuous unrealized losses existing greater than 12 months, gross 4,000    
Total 76,000 33,000  
Mutual funds and other equity securities      
Available for sale      
Amortized Cost 4,944,000 4,114,000  
Gross Unrealized Gains 1,401,000 1,752,000  
Fair Value 6,345,000 5,866,000  
Commercial mortgage-backed securities      
Held to maturity      
Amortized Cost 1,014,000    
Gross Unrealized Gains 40,000    
Fair Value 1,054,000    
Held to maturity, Amortized Cost      
Amortized Cost 1,014,000    
Held to maturity, Fair Value      
Fair Value $ 1,054,000