Other Comprehensive Income - Summary of Change in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Unrealized gains (losses) on securities available for sale, Beginning Balance	$ 2,736	$ (7,636)	$ 16,039
Unrealized gains (losses) on securities available for sale, Increase (Decrease)	793	10,372	(23,675)
Unrealized gains (losses) on securities available for sale, Ending Balance	3,529	2,736	(7,636)
Unrealized gain (loss) on postretirement benefits plan, Increase (Decrease)	(1,063)	(1,084)	(981)
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(90)	21	(103)
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(1,153)	(1,063)	(1,084)
Unrealized gain (loss) on interest rate swap, Beginning Balance	(614)	(725)	(1,134)
Unrealized gain (loss) on interest rate swap, Increase (Decrease)	137	111	409
Unrealized gain (loss) on interest rate swap, Ending Balance	(477)	(614)	(725)
Total, Beginning Balance	1,059	(9,445)	13,924
Total, Increase (Decrease)	840	10,504	(23,369)
Total, Ending Balance	$ 1,899	$ 1,059	$ (9,445)