SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Aug. 14, 2019 USD ($) $ / shares shares	Jun. 30, 2016 shares	Dec. 31, 2019 USD ($) company item state shares	Dec. 31, 2018 shares	Aug. 13, 2019
Number of passive investment subsidiaries | item			1
Net proceeds from sale of common stock	$ 310,400		$ 304,061
Expenses incurred	6,300		6,300
Net proceeds from sale of common stock	$ 304,100
Shares issued | shares	31,036,812
Shares issued (in dollars per share) | $ / shares	$ 10.00
Exchange Ratio Applied In Conversion Offering	1.795431
Shares issued in exchange | shares	12,162,763
Amount utilized to fund new ESOP	$ 24,800		$ 24,829
Number of full-service bank offices | item			25
Number of limited-service bank offices | item			1
Impairment of federal home loan bank stock			$ 0
Number of reporting units | item			2
Number of security corporations | item			2
Impairment recognized of goodwill and intangible assets			$ 0
Shares repurchased | shares			71,201	146,644
Shares acquired in connection with satisfaction of tax obligations | shares				75,544
Cash | U. S. Bancorp Deposit
Concentration risk, amount			$ 128,600
Cash | Federal Reserve Bank Deposit
Concentration risk, amount			$ 52,500
HarborOne Mortgage
Number of offices | item			36
Additional states licensed to lend | state			4
HarborOne Foundation
Shares issued | shares		385,450
HarborOne Bank
Number of security corporation subsidiaries | company			2
Cash invested into Bank's operations	$ 151,300
HarborOne Bancorp Inc. | MHC
Ownership percentage					53.00%
Residential | LTV 80 to 100 Percent
Loan To Value Ratio			80.00%