Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ (26)	$ (14)	$ (40)	$ (21)
Interest income	62,685	62,574	124,851	124,091
Salaries and employee benefits	30,831	31,142	61,013	60,676
Income tax expense (benefit)	9,757	14,059	18,947	22,731
Net of tax	19,281	27,640	37,447	45,649
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	531	3	1,070	10
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]
Net securities gains	(26)	(14)	(40)	(21)
Interest income	307	0	614	0
Income tax expense (benefit)	(92)	6	(188)	8
Net of tax	189	(8)	386	(13)
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)	(219)	(8)	(438)	(15)
Net of tax	342	11	684	23
Amortization of Net Gains [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	567	74	1,133	148
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	$ (6)	$ (55)	$ (11)	$ (110)