Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (gains) losses	$ (1,867)	$ 644	$ (3,087)
Interest income	283,493	264,441	252,608
Other noninterest income	11,991	15,961	14,194
Salaries and employee benefits	133,610	131,284	125,633
Income tax (expense) benefit	46,010	40,392	40,203
Net Income	82,151	78,409	76,425
Total reclassifications, net of tax	1,310	2,482	286
Accumulated Other Comprehensive (Loss) Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	0	0	0
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	(120)	(677)	691
Net Income	198	1,061	(1,085)
Gains (Losses) on Available for Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (gains) losses	(1,869)	644	(3,087)
Amortization of Unrealized Gains and Losses Related to Securities Transfer [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	875	1,094	1,311
Impairment Write-Down of Equity Security [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other noninterest income	1,312	0	0
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	(740)	(949)	(868)
Net Income	1,112	1,421	1,371
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	1,755	2,395	2,437
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	$ 97	$ (25)	$ (198)