SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Millions		3 Months Ended
	Aug. 14, 2019 USD ($) $ / shares shares	Mar. 31, 2020 company Office item state shares	Mar. 31, 2019	Dec. 31, 2019 shares	Aug. 13, 2019
Net proceeds from sale of common stock | $	$ 310.4
Expenses incurred | $	6.3
Net proceeds from sale of common stock | $	$ 304.1
Shares issued | shares	31,036,812
Shares issued (in dollars per share) | $ / shares	$ 10.00
Stock conversion ratio	1.795431	1.795431	1.795431
Shares issued in exchange | shares	12,162,763
Number of full-service bank offices | item		25
Number of limited-service bank offices | item		1
Shares repurchased | shares		71,201		71,201
HarborOne Bank
Number of security corporation subsidiaries | company		2
HarborOne Mortgage
Number of offices | Office		30
Additional states licensed to lend | state		4
HarborOne Bancorp Inc. | MHC
Ownership percentage					53.00%
Residential | LTV 80 to 100 Percent
Loan To Value Ratio		80.00%