Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (losses) gains	$ 644	$ (3,087)	$ (92)
Interest income	264,441	252,608	251,878
Salaries and employee benefits	129,702	124,318	119,667
Income tax expense	40,392	40,203	37,229
Net Income	78,409	76,425	$ 75,074
Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassification during the period, net of tax	2,482	286
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]
Income tax expense	(677)	691
Net Income	1,061	(1,085)
Gains on Available for Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (losses) gains	644	(3,087)
Amortization of Unrealized Gains and Losses Related to Securities Transfer [Member] | Reclassification out of Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	1,094	1,311
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	949	868
Net Income	1,421	1,371
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	2,395	2,437
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	$ (25)	$ (198)