Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ 1	$ 26	$ 30	$ 40
Interest income	65,777	62,685	130,354	124,851
Salaries and employee benefits	32,931	30,831	65,372	61,013
Income tax expense	(10,257)	(9,757)	(19,988)	(18,947)
Net income	19,909	19,281	38,800	37,447
Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	484	531	959	1,070
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	(1)	(26)	(30)	(40)
Interest income	280	336	576	671
Income tax expense	(108)	(121)	(213)	(245)
Net income	171	189	333	386
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	(199)	(219)	(398)	(438)
Net income	313	342	626	684
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	515	567	1,030	1,133
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	$ (3)	$ (6)	$ (6)	$ (11)