Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Allowance for loan losses	$ 13,114	$ 13,251
Post-retirement benefits	4,089	3,720
Deferred compensation	2,085	2,228
Depreciation	1,002	1,042
Retirement Income Maintenance plan	1,725	1,602
ESOP	301	128
Stock-based compensation	788	35
Reserve for uncollected interest	54	24
Net unrealized losses on debt securities and defined benefit plans	18,374	19,060
Federal and State NOLs	8,333	2,063
Alternative minimum assessment carryforwards	2,156	2,156
Charitable contribution carryforward	5,981	6,085
Purchase accounting	665	0
Other items	332	670
Gross deferred tax assets	58,999	52,064
Valuation allowance	(7,442)	(2,388)
Total deferred tax assets, net	51,557	49,676
Deferred tax liabilities:
Pension expense	33,856	26,071
Deferred loan costs	5,911	5,736
Intangible assets	1,215	1,621
Other items	266	34
Total gross deferred tax liabilities	41,248	33,462
Net deferred tax asset	$ 10,309	$ 16,214