Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive (Loss) Income, Net of Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance	$ (19,454)	$ (16,708)	$ (14,462)
Net Change	1,683	(2,746)	(2,246)
Ending balance	(17,771)	(19,454)	(16,708)
Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance	(4,988)	(592)	1,711
Net Change	927	(4,396)	(2,303)
Ending balance	(4,061)	(4,988)	(592)
Net Change Related to Derivative Used for Cash Flow Hedge [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance	(141)	(285)	(157)
Net Change	150	144	(128)
Ending balance	9	(141)	(285)
Net Change Related to Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance	(14,325)	(15,831)	(16,016)
Net Change	606	1,506	185
Ending balance	$ (13,719)	$ (14,325)	$ (15,831)