Other Comprehensive Income - Summary of Change in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 243,910	$ 211,784	$ 201,913
Comprehensive Income (Loss)	(3,653)	15	(2,020)
Reclassification of Disproportionate Tax Effect		0
Ending balance	262,830	243,910	211,784
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(343)	(121)	1,899
Comprehensive Income (Loss)	(3,653)	15	(2,020)
Reclassification of Disproportionate Tax Effect	0	(237)	0
Ending balance	(3,996)	(343)	(121)
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	473	1,393	3,529
Comprehensive Income (Loss)	(4,007)	(1,004)	(2,136)
Reclassification of Disproportionate Tax Effect	0	84	0
Ending balance	(3,534)	473	1,393
Unrealized gain (loss) on postretirement benefits plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(690)	(1,217)	(1,153)
Comprehensive Income (Loss)	386	825	(64)
Reclassification of Disproportionate Tax Effect	0	(298)	0
Ending balance	(304)	(690)	(1,217)
Unrealized loss on interest rate swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(126)	(297)	(477)
Comprehensive Income (Loss)	(32)	194	180
Reclassification of Disproportionate Tax Effect	0	(23)	0
Ending balance	$ (158)	$ (126)	$ (297)