INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses and off-balance sheet credit exposure	$ 13,983	$ 10,305
Net unrealized losses on securities		343
Compensation and related benefit obligations	1,674	2,368
Net deferred loan costs and fees	2,588
Purchase accounting fair value adjustments	3,574	4,735
Net change in pension and other post-retirement benefits plans	3,608	2,809
Net operating loss carryforward	786	3,229
Net loss on cash flow hedges	1,905	304
Operating lease liabilities	13,684	13,444
Other	1,234	200
Total deferred tax assets	43,036	37,737
Deferred tax liabilities:
Pension and SERP expense	(5,366)	(4,904)
Net unrealized gains on securities	(1,285)
Depreciation	(546)	(956)
REIT undistributed net income	(3,178)	(2,403)
Net deferred loan costs and fees		(2,413)
State and local taxes	(1,345)	(1,227)
Operating lease right-of-use assets	(13,172)	(12,934)
Other	(970)	(835)
Total deferred tax liabilities	(25,862)	(25,672)
Net deferred tax asset	$ 17,174	$ 12,065