MINIMUM REGULATORY CAPITAL REQUIREMENTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital conversation buffer ratio	2.50%
Applicable capital conversation buffer ratio	2.50%
Common equity Tier 1 to risk-weighted assets [Abstract]
Actual, Capital amount	$ 596,997	$ 590,122
Actual, Ratio (as a percent)	18.50%	18.70%
Minimum Requirement for Capital Adequacy Purposes	$ 145,224	$ 142,048
Minimum Requirement for Capital Adequacy Purposes (as a percent)	4.50%	4.50%
Tier 1 capital to risk weighted assets
Actual, Capital amount	$ 596,997	$ 590,122
Actual, Ratio (as a percent)	18.50%	18.70%
Minimum Requirement for Capital Adequacy Purposes	$ 193,633	$ 189,397
Minimum Requirement for Capital Adequacy Purposes (as a percent)	6.00%	6.00%
Total capital to risk-weighted assets
Actual, Capital amount	$ 658,387	$ 649,182
Actual, Ratio (as a percent)	20.40%	20.60%
Minimum Requirement for Capital Adequacy Purposes	$ 258,177	$ 252,529
Minimum Requirement for Capital Adequacy Purposes (as a percent)	8.00%	8.00%
Tier 1 capital to average assets
Actual, Capital amount	$ 596,997	$ 590,122
Actual, Ratio (as a percent)	15.20%	15.30%
Minimum Requirement for Capital Adequacy Purposes	$ 157,143	$ 154,659
Minimum Requirement for Capital Adequacy Purposes (as a percent)	4.00%	4.00%
HarborOne Bank
Common equity Tier 1 to risk-weighted assets [Abstract]
Actual, Capital amount	$ 460,475	$ 453,707
Actual, Ratio (as a percent)	14.40%	14.40%
Minimum Requirement for Capital Adequacy Purposes	$ 144,082	$ 142,053
Minimum Requirement for Capital Adequacy Purposes (as a percent)	4.50%	4.50%
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 208,118	$ 205,188
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions (as a percent)	6.50%	6.50%
Tier 1 capital to risk weighted assets
Actual, Capital amount	$ 460,475	$ 453,707
Actual, Ratio (as a percent)	14.40%	14.40%
Minimum Requirement for Capital Adequacy Purposes	$ 192,109	$ 189,404
Minimum Requirement for Capital Adequacy Purposes (as a percent)	6.00%	6.00%
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 256,146	$ 252,539
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions (as a percent)	8.00%	8.00%
Total capital to risk-weighted assets
Actual, Capital amount	$ 486,864	$ 477,767
Actual, Ratio (as a percent)	15.20%	15.10%
Minimum Requirement for Capital Adequacy Purposes	$ 256,146	$ 252,539
Minimum Requirement for Capital Adequacy Purposes (as a percent)	8.00%	8.00%
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 320,182	$ 315,674
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions (as a percent)	10.00%	10.00%
Tier 1 capital to average assets
Actual, Capital amount	$ 460,475	$ 453,707
Actual, Ratio (as a percent)	12.10%	12.20%
Minimum Requirement for Capital Adequacy Purposes	$ 151,661	$ 149,272
Minimum Requirement for Capital Adequacy Purposes (as a percent)	4.00%	4.00%
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 189,576	$ 186,591
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions (as a percent)	5.00%	5.00%