Other Comprehensive Income - Summary of Change in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Unrealized gains (losses) on securities available for sale, Beginning Balance	$ 3,529	$ 2,736	$ (7,636)
Unrealized gains (losses) on securities available for sale, Increase (Decrease)	(2,136)	793	10,372
Unrealized gains (losses) on securities available for sale, Ending Balance	1,393	3,529	2,736
Unrealized gain (loss) on postretirement benefits plan, Increase (Decrease)	(1,153)	(1,063)	(1,084)
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(64)	(90)	21
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(1,217)	(1,153)	(1,063)
Unrealized gain (loss) on interest rate swap, Beginning Balance	(477)	(614)	(725)
Unrealized gain (loss) on interest rate swap, Increase (Decrease)	180	137	111
Unrealized gain (loss) on interest rate swap, Ending Balance	(297)	(477)	(614)
Total, Beginning Balance	1,899	1,059	(9,445)
Total, Increase (Decrease)	(2,020)	840	10,504
Total, Ending Balance	$ (121)	$ 1,899	$ 1,059