Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (29)	$ (14)
Interest income	64,577	62,166
Salaries and employee benefits	32,441	30,182
Income tax expense	9,731	9,190
Net of tax	18,891	18,166
Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	475	537
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	(29)	(14)
Interest income	296	307
Income tax expense	(105)	(98)
Net of tax	162	195
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)
Net of tax	313	342
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	566
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)	$ (5)