COMPENSATION AND BENEFIT PLANS (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) item shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Benefit plan liability	$ 1,953,000	$ 1,915,000	$ 1,876,000	$ 1,837,000
Gain on settlements		43,000
Contributions expenses	$ 1,800,000	$ 2,500,000	$ 2,300,000
Allocated shares | shares	639,525	408,803
Shares committed to be allocated | shares	230,723	230,724	106,576
Unallocated shares | shares	3,602,562	3,833,283
Total shares | shares	4,472,810	4,472,810
Fair value of unallocated shares, end of period	$ 39,123,823	$ 42,127,780
ESOP compensation expense	$ 2,103,000	2,655,000	$ 1,097,000
ESOP restoration benefit payable period	six months and a day
Restoration accrual	$ 487,000	$ 210,000	$ 24,000
Through 2035
Defined Benefit Plan Disclosure [Line Items]
Shares committed to be allocated | shares	230,723
From 2036 to 2038
Defined Benefit Plan Disclosure [Line Items]
Shares committed to be allocated | shares	124,148
HarborOne Bank
Defined Benefit Plan Disclosure [Line Items]
Employee compensation the bank contributed (as a percent)	5.00%	9.30%	9.30%
Employee's compensation in exess of social security wage match (as a percent)		5.70%	5.70%
HarborOne Mortgage
Defined Benefit Plan Disclosure [Line Items]
Percentage of matched contribution	50.00%
Employee contributions matched 50% (as a percent)	4.00%
Maximum amount employer will match		$ 0	$ 0
Maximum | HarborOne Mortgage
Defined Benefit Plan Disclosure [Line Items]
Maximum amount employer will match	$ 2,000
Split-dollar Life Insurance Arrangements
Defined Benefit Plan Disclosure [Line Items]
Expense amount	$ 21,000	38,000	73,000
Deferred Compensation Plan
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	1
Interest rate (as a percent)	1.50%
Expense amount	$ 32,000	31,000	29,000
Eligible age for medical insurance plan	65 years
Deferred Compensation Plan | Coastway
Defined Benefit Plan Disclosure [Line Items]
Expense amount			46,000
Deferred Compensation Plan | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 385,000	353,000	322,000
Deferred Compensation Plan | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Benefit plan liability			1,500,000
Deferred Compensation Plan | Other assets | Coastway
Defined Benefit Plan Disclosure [Line Items]
Plan Assets			1,500,000
Supplemental medical and prescription drug
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	2
Expense amount	$ 47,000	66,000	3,000
Supplemental medical and prescription drug | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	344,000	297,000	229,000
Management Incentive Program
Defined Benefit Plan Disclosure [Line Items]
Expense amount	4,700,000	2,100,000	3,300,000
Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions expenses	$ 955,000	1,800,000	658,000
Number of SERP agreements | item	2
Supplemental Retirement Plans | Coastway
Defined Benefit Plan Disclosure [Line Items]
Expense amount			90,000
Supplemental Retirement Plans | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 8,000,000.0	7,100,000	5,300,000
Supplemental Retirement Plans | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Employer obligation			3,400,000
Supplemental Retirement Plans | Executive
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	2
Long-Term Incentive Plan
Defined Benefit Plan Disclosure [Line Items]
Expense amount	$ 20,000		56,000
Long-Term Incentive Plan | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	437,000	838,000	1,300,000
Long-Term Incentive Plan | Executive
Defined Benefit Plan Disclosure [Line Items]
Expense amount		(82,000)
Post-Retirement Life Insurance
Defined Benefit Plan Disclosure [Line Items]
Expense amount	47,000	17,000	29,000
Post-Retirement Life Insurance | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 234,000	$ 186,000	171,000
Directors Retirement Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Vesting period	6 years
Defined benefit pension plan | Coastway
Defined Benefit Plan Disclosure [Line Items]
Plan Assets			1,400,000
Benefit plan liability			1,600,000
Defined benefit pension plan | Unfunded Plan | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Benefit plan liability			$ 202,000