Accumulated Other Comprehensive Loss - Amounts Reclassified Out of Each Component of Accumulated Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain on sales of investment securities									$ 1,265	$ 635	$ 3,389
Other-than-temporary impairment on equity securities									0
Amortization of net loss included in net periodic pension costs									1,362	666	1,282
Accretion of prior service cost included in net periodic pension costs									(280)	(288)	(255)
Income before income taxes	$ 9,711	$ 10,307	$ 8,758	$ 8,250	$ 6,822	$ 8,499	$ 6,627	$ 7,731	37,026	29,679	26,885
Tax (expense) benefit	(2,553)	(2,779)	(2,292)	(2,134)	(1,632)	(2,264)	(1,547)	(2,005)	(9,758)	(7,448)	(5,696)
Net income	$ 7,158	$ 7,528	$ 6,466	$ 6,116	$ 5,190	$ 6,235	$ 5,080	$ 5,726	27,268	22,231	21,189
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									1,265	635	3,389
Other-than-temporary impairment on equity securities									(5)	0	0
Income before income taxes									1,260	635	3,389
Tax (expense) benefit									(441)	(222)	(1,186)
Net income									819	413	2,203
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	0	(1,866)
Income before income taxes									0	0	(1,866)
Tax (expense) benefit									0	0	653
Net income									0	0	(1,213)
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									(1,362)	(666)	(1,282)
Accretion of prior service cost included in net periodic pension costs									280	288	255
Income before income taxes									(1,082)	(378)	(1,027)
Tax (expense) benefit									379	132	360
Net income									$ (703)	$ (246)	$ (667)