Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	$ 413	$ 181	$ 457	$ 272
Amortization of net loss included in net periodic pension costs	329	340	658	681
Accretion of prior service cost included in net periodic pension costs	(70)	(70)	(141)	(140)
Income before income taxes	7,286	8,758	17,375	17,008
Tax (expense) benefit	(2,046)	(2,292)	(4,846)	(4,426)
Net income	5,240	6,466	12,529	12,582
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	413	181	457	272
Income before income taxes	413	181	457	272
Tax (expense) benefit	(145)	(63)	(160)	(95)
Net income	268	118	297	177
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Change Related to Defined Benefit Pension Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss included in net periodic pension costs	(329)	(340)	(658)	(681)
Accretion of prior service cost included in net periodic pension costs	70	70	141	140
Income before income taxes	(259)	(270)	(517)	(541)
Tax (expense) benefit	91	94	181	190
Net income	$ (168)	$ (176)	$ (336)	$ (351)