Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses on loans	$ 26,156	$ 14,313
Allowance for credit loss OBS	1,295	0
Post-retirement benefit	6,924	6,946
Deferred compensation	839	1,175
Contingent Consideration	714	0
Purchase accounting adjustments	3,099	1,629
Depreciation	331	750
SERP	733	688
ESOP	1,377	1,606
Stock-based compensation	4,481	4,747
Payroll Protection Program fees	2,268	0
Non-accrual interest	225	417
Federal Net Operating Loss ("NOL")	280	321
Net unrealized loss on hedging activities	1,717	0
Pension liability adjustments	376	1,821
Other	925	1,223
Total gross deferred tax assets	51,740	35,636
Deferred tax liabilities:
Pension expense	7,340	7,017
Deferred loan costs	4,532	5,064
Investment securities, principally due to accretion of discounts	79	70
Intangibles	1,723	1,393
Originated mortgage servicing rights	160	140
Unrealized gain on available for sale debt securities	7,802	3,038
Net unrealized gain on hedging activities	0	114
Total gross deferred tax liabilities	21,636	16,836
Net deferred tax asset	$ 30,104	$ 18,800