Other Comprehensive Income - Summary of Change in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Unrealized gains (losses) on securities available for sale, Beginning Balance	$ 1,393	$ 3,529	$ 2,736
Unrealized gains (losses) on securities available for sale, Comprehensive Income (Loss)	(1,004)	(2,136)	793
Unrealized gains (losses) on securities available for sale, Reclassification of Disproportionate Tax Effect	84	0	0
Unrealized gains (losses) on securities available for sale, Ending Balance	473	1,393	3,529
Unrealized gain (loss) on postretirement benefits plan, Beginning Balance	(1,217)	(1,153)	(1,063)
Unrealized gain (loss) on postretirement benefits plan, Comprehensive Income (Loss)	825	(64)	(90)
Unrealized gain (loss) on postretirement benefits plan, Reclassification of Disproportionate Tax Effect	(298)	0	0
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(690)	(1,217)	(1,153)
Unrealized loss on interest rate swap, Beginning Balance	(297)	(477)	(614)
Unrealized loss on interest rate swap, Comprehensive Income (Loss)	194	180	137
Unrealized loss on interest rate swap, Reclassification of Disproportionate Tax Effect	(23)	0	0
Unrealized loss on interest rate swap, Ending Balance	(126)	(297)	(477)
Total, Beginning Balance	(121)	1,899	1,059
Total, Comprehensive Income (Loss)	15	(2,020)	840
Total, Reclassification of Disproportionate Tax Effect	(237)	0	0
Total, Ending Balance	$ (343)	$ (121)	$ 1,899