INCOME TAXES - Deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Allowance for loan losses	$ 6,651	$ 5,776
Employee benefit plans	5,036	5,288
Mark-to-market loans	3,687	4,728
Accrued expenses not deducted for tax purposes	1,093	976
HarborOne Mortgage loan repurchase reserve	231	212
Charitable contribution and other carryforwards	15	380
Net unrealized loss on securities available for sale		665
Other		661
Total deferred tax assets	16,713	18,686
Deferred tax liabilities:
Net unrealized gain on securities available for sale	(419)
Deferred income annuities	(1,447)	(1,352)
Depreciation and amortization	(370)	(294)
Deferred loan fees	(1,995)	(1,742)
Mortgage servicing rights	(4,759)	(6,241)
Core deposit intangible	(1,668)	(2,330)
Other	(21)
Total deferred tax liabilities	(10,679)	(11,959)
Net deferred tax asset	$ 6,034	$ 6,727	$ 843	$ 610