Reclassification Adjustments Out of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ 4	$ 0	$ 2	$ (30)
Interest income	71,930	66,662	210,042	197,016
Other noninterest income	3,945	4,113	10,069	12,188
Salaries and employee benefits	32,740	32,783	99,081	98,155
Income tax (expense)	(11,342)	(10,303)	(30,321)	(30,291)
Net income	22,876	20,001	64,514	58,801
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassification during the period, net of tax	430	479	2,063	1,439
AOCI Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income			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)	(83)	(105)	(760)	(317)
Net income	133	162	1,229	496
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	4	0	2	(30)
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	212	267	675	843
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
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)	(183)	(202)	(516)	(601)
Net income	297	317	834	943
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	481	565	1,351	1,595
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	$ (1)	$ (46)	$ (1)	$ (51)