Accumulated Other Comprehensive Loss - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities									$ 635	$ 3,389	$ 305
Other-than-temporary impairment on equity securities									0	0	(13)
Amortization of net loss included in net periodic pension costs									666	1,282	1,170
Accretion of prior service cost included in net periodic pension costs									(288)	(255)	(255)
Income before income taxes	6,822	8,499	6,627	7,731	5,971	7,439	6,367	7,108	29,679	26,885	26,423
Tax (expense) benefit	(1,632)	(2,264)	(1,547)	(2,005)	(1,049)	(1,400)	(1,537)	(1,710)	(7,448)	(5,696)	(5,551)
Net income	5,190	6,235	5,080	5,726	4,922	6,039	4,830	5,398	22,231	21,189	20,872
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									635	3,389	305
Other-than-temporary impairment on equity securities									0	0	(13)
Income before income taxes									635	3,389	292
Tax (expense) benefit									(222)	(1,186)	(102)
Net income									413	2,203	190
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income [Member] | Net Change Related to Derivative Used for Cash Flow Hedge [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net loss on interest rate swap									0	(1,866)	0
Income before income taxes									0	(1,866)	0
Tax (expense) benefit									0	653	0
Net income									0	(1,213)	0
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									(666)	(1,282)	(1,170)
Accretion of prior service cost included in net periodic pension costs									288	255	255
Income before income taxes									(378)	(1,027)	(915)
Tax (expense) benefit									132	360	320
Net income									$ (246)	$ (667)	$ (595)