Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities	$ 296	$ 0	$ 568	$ 557
Amortization of net loss included in net periodic pension costs	341	168	1,022	499
Accretion of prior service cost included in net periodic pension costs	(70)	(72)	(210)	(216)
Income before income taxes	10,307	8,499	27,315	22,857
Tax (expense) benefit	(2,779)	(2,264)	(7,205)	(5,816)
Net income	7,528	6,235	20,110	17,041
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	296	0	568	557
Other-than-temporary impairment on equity securities	(5)	0	(5)	0
Income before income taxes	291	0	563	557
Tax (expense) benefit	(102)	0	(197)	(195)
Net income	189	0	366	362
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	(341)	(168)	(1,022)	(499)
Accretion of prior service cost included in net periodic pension costs	70	72	210	216
Income before income taxes	(271)	(96)	(812)	(283)
Tax (expense) benefit	95	33	285	98
Net income	$ (176)	$ (63)	$ (527)	$ (185)