Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ 14	$ 7
Interest income	62,166	61,517
Income tax expense	9,190	8,672
Net of tax	18,166	18,009
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	537	7
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	(14)	(7)
Interest income	307	0
Income tax expense	(98)	3
Net of tax	195	(4)
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	218	8
Net of tax	342	11
Amortization of Net Gains [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	565	74
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	$ (5)	$ (55)