Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	$ 44	$ 91
Amortization of net loss included in net periodic pension costs	329	341
Accretion of prior service cost included in net periodic pension costs	(71)	(70)
Income before income taxes	10,089	8,250
Tax (expense) benefit	(2,800)	(2,134)
Net income	7,289	6,116
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	44	91
Income before income taxes	44	91
Tax (expense) benefit	(15)	(32)
Net income	29	59
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)	(341)
Accretion of prior service cost included in net periodic pension costs	71	70
Income before income taxes	(258)	(271)
Tax (expense) benefit	90	96
Net income	$ (168)	$ (175)