MINIMUM REGULATORY CAPITAL REQUIREMENTS (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations
Common equity Tier 1 capital conversation buffer ratio	0.025
Applicable capital conversation buffer ratio	0.025
Common equity Tier 1 to risk-weighted assets
Actual, Capital amount	$ 608,882	$ 621,153
Actual, Ratio	0.171	0.177
Minimum Requirement for Capital Adequacy Purposes	$ 160,129	$ 158,050
Minimum Requirement for Capital Adequacy Purposes, ratio	0.045	0.045
Tier 1 capital to risk weighted assets
Actual, Capital amount	$ 608,882	$ 621,153
Actual, Ratio	0.171	0.177
Minimum Requirement for Capital Adequacy Purposes	$ 213,505	$ 210,733
Minimum Requirement for Capital Adequacy Purposes, ratio	0.060	0.060
Total capital to risk-weighted assets
Actual, Capital amount	$ 688,405	$ 700,197
Actual, Ratio	0.193	0.199
Minimum Requirement for Capital Adequacy Purposes	$ 284,673	$ 280,978
Minimum Requirement for Capital Adequacy Purposes, ratio	0.080	0.080
Tier 1 capital to average assets
Actual, Capital amount	$ 608,882	$ 621,153
Actual, Ratio	0.135	0.145
Minimum Requirement for Capital Adequacy Purposes	$ 180,139	$ 171,578
Minimum Requirement for Capital Adequacy Purposes, ratio	0.040	0.040
HarborOne Bank
Common equity Tier 1 to risk-weighted assets
Actual, Capital amount	$ 559,819	$ 506,822
Actual, Ratio	0.157	0.144
Minimum Requirement for Capital Adequacy Purposes	$ 160,182	$ 158,081
Minimum Requirement for Capital Adequacy Purposes, ratio	0.045	0.045
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 231,374	$ 228,339
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions, ratio	0.065	0.065
Tier 1 capital to risk weighted assets
Actual, Capital amount	$ 559,819	$ 506,822
Actual, Ratio	0.157	0.144
Minimum Requirement for Capital Adequacy Purposes	$ 213,576	$ 210,775
Minimum Requirement for Capital Adequacy Purposes, ratio	0.060	0.060
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 284,768	$ 281,033
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions, ratio	0.080	0.080
Total capital to risk-weighted assets
Actual, Capital amount	$ 604,357	$ 550,875
Actual, Ratio	0.170	0.157
Minimum Requirement for Capital Adequacy Purposes	$ 284,768	$ 281,033
Minimum Requirement for Capital Adequacy Purposes, ratio	0.080	0.080
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 355,960	$ 351,291
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions, ratio	0.100	0.100
Tier 1 capital to average assets
Actual, Capital amount	$ 559,819	$ 506,822
Actual, Ratio	0.124	0.118
Minimum Requirement for Capital Adequacy Purposes	$ 180,113	$ 171,501
Minimum Requirement for Capital Adequacy Purposes, ratio	0.040	0.040
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions	$ 225,141	$ 214,377
Minimum Required to be Considered "Well Capitalized" Under Prompt Corrective Action Provisions, ratio	0.050	0.050