INCOME TAXES - Deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowance for loan losses	$ 15,493	$ 6,651
Employee benefit plans	4,999	5,036
Mark-to-market loans	2,387	3,687
Accrued expenses not deducted for tax purposes	1,799	1,093
HarborOne Mortgage loan repurchase reserve	743	231
Charitable contribution and other carryforwards		15
Total deferred tax assets	25,421	16,713
Deferred tax liabilities:
Net unrealized gain on securities available for sale	(904)	(419)
Derivatives	(2,349)
Deferred income annuities	(1,580)	(1,447)
Depreciation and amortization	(1,145)	(370)
Deferred loan fees	(1,578)	(1,995)
Mortgage servicing rights	(6,793)	(4,759)
Core deposit intangible	(1,222)	(1,668)
Other	(293)	(21)
Total deferred tax liabilities	(15,864)	(10,679)
Net deferred tax asset	$ 9,557	$ 6,034	$ 6,727	$ 843