Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Allowance for loan losses	$ 14,313	$ 13,968
Post-retirement benefit	6,946	7,481
Deferred compensation	1,175	1,371
Purchase accounting adjustments	1,629	1,562
Depreciation	750	215
SERP	688	694
ESOP	1,606	1,929
Stock-based compensation	4,747	4,464
Non-accrual interest	417	867
Unrealized loss on available for sale debt securities	0	3,599
Federal Net Operating Loss ("NOL")	321	363
Pension liability adjustments	1,821	1,358
Other	1,223	2,164
Total gross deferred tax assets	35,636	40,035
Deferred tax liabilities:
Pension expense	7,017	7,322
Deferred loan costs	5,064	4,872
Investment securities, principally due to accretion of discounts	70	93
Intangibles	1,393	1,159
Originated mortgage servicing rights	140	165
Unrealized gain on available for sale debt securities	3,038	0
Net unrealized gain on hedging activities	114	0
Total gross deferred tax liabilities	16,836	13,611
Net deferred tax asset	$ 18,800	$ 26,424