Securities (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Available for sale
Amortized Cost	$ 891,383		$ 749,364
Gross Unrealized Gains	11,875		10,740
Gross Unrealized Losses	(492)		(1,039)
Fair Value	902,766		759,065
Held to maturity
Amortized Cost	49,812		2,373
Gross Unrealized Gains	505		55
Gross Unrealized Losses	(29)		(3)
Fair Value	50,288		2,425
Available for sale, Amortized Cost
Due in one year or less	107,516
Due after one year through five years	417,157
Due after five years through ten years	112,909
Due after ten years	242,428
Amortized Cost	880,010
Available for sale, Fair Value
Due in one year or less	107,924
Due after one year through five years	420,229
Due after five years through ten years	116,344
Due after ten years	246,029
Fair Value	890,526
Held to maturity, Amortized Cost
Due in one year or less	1,053
Due after one year through five years	13,845
Due after five years through ten years	27,764
Due after ten years	7,150
Amortized Cost	49,812		2,373
Held to maturity, Fair Value
Due in one year or less	1,055
Due after one year through five years	13,955
Due after five years through ten years	28,056
Due after ten years	7,222
Fair Value	50,288		2,425
Realized gains and losses related to sales of securities available for sale
Gross security gains	1	$ 57
Gross security (losses)	(22)	(17)
Net security (losses) gains	(21)	$ 40
Carrying amount of investment securities pledged as collateral	606,800		536,200
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	77,039		67,415
Continuous unrealized losses existing greater than 12 months, gross	11,478		65,898
Total	88,517		133,313
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(388)		(234)
Continuous unrealized losses existing greater than 12 months, gross	(104)		(805)
Total	(492)		(1,039)
Held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	5,188		534
Total	5,188		534
Held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	29		3
Total	$ 29		3
Number of securities in unrealized loss position | item	87
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.60%
U.S. Treasury securities
Available for sale
Amortized Cost	$ 65,062		50,280
Gross Unrealized Gains	701		328
Gross Unrealized Losses			(2)
Fair Value	65,763		50,606
Available for sale, Fair Value
Continuous unrealized losses existing greater than 12 months, gross			366
Total			366
Available for sale, Unrealized Losses
Continuous unrealized losses existing greater than 12 months, gross			(2)
Total			(2)
Obligations of U.S. government corporations and agencies
Available for sale
Amortized Cost	149,620		166,207
Gross Unrealized Gains	813		981
Gross Unrealized Losses			(178)
Fair Value	150,433		167,010
Available for sale, Fair Value
Continuous unrealized losses existing greater than 12 months, gross			25,118
Total			25,118
Available for sale, Unrealized Losses
Continuous unrealized losses existing greater than 12 months, gross			(178)
Total			(178)
Obligations of states and political subdivisions
Available for sale
Amortized Cost	189,953		218,250
Gross Unrealized Gains	2,801		2,672
Gross Unrealized Losses	(157)		(761)
Fair Value	192,597		220,161
Held to maturity
Amortized Cost	48,810		1,359
Gross Unrealized Gains	457		15
Gross Unrealized Losses	(29)		(3)
Fair Value	49,238		1,371
Held to maturity, Amortized Cost
Amortized Cost	48,810		1,359
Held to maturity, Fair Value
Fair Value	49,238		1,371
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	8,613		40,385
Continuous unrealized losses existing greater than 12 months, gross	11,478		40,201
Total	20,091		80,586
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(53)		(140)
Continuous unrealized losses existing greater than 12 months, gross	(104)		(621)
Total	(157)		(761)
Held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	5,188		534
Total	5,188		534
Held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	29		3
Total	29		3
Residential mortgage-backed Securities
Available for sale
Amortized Cost	322,919		230,596
Gross Unrealized Gains	5,930		5,062
Gross Unrealized Losses	(17)		(22)
Fair Value	328,832		235,636
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	6,412		10,630
Total	6,412		10,630
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(17)		(22)
Total	(17)		(22)
Corporate debt securities
Available for sale
Amortized Cost	152,456		79,087
Gross Unrealized Gains	763		296
Gross Unrealized Losses	(318)		(76)
Fair Value	152,901		79,307
Available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	62,014		16,400
Continuous unrealized losses existing greater than 12 months, gross			213
Total	62,014		16,613
Available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(318)		(72)
Continuous unrealized losses existing greater than 12 months, gross			(4)
Total	(318)		(76)
Debt securities
Available for sale
Amortized Cost	880,010		744,420
Gross Unrealized Gains	11,008		9,339
Gross Unrealized Losses	(492)		(1,039)
Fair Value	890,526		752,720
Mutual funds and other equity securities
Available for sale
Amortized Cost	11,373		4,944
Gross Unrealized Gains	867		1,401
Fair Value	12,240		6,345
Commercial mortgage-backed securities
Available for sale
Amortized Cost			1,014
Gross Unrealized Gains			40
Fair Value			$ 1,054
Held to maturity
Amortized Cost	1,002
Gross Unrealized Gains	48
Fair Value	1,050
Held to maturity, Amortized Cost
Amortized Cost	1,002
Held to maturity, Fair Value
Fair Value	$ 1,050