SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment	2
Net interest and dividend income (expense)	$ 26,700	$ 26,030
Provision for loan losses	3,749	857
Net interest and dividend income, after provision for loan losses	22,951	25,173
Mortgage banking income:
Changes in mortgage servicing rights fair value	(4,387)	(2,151)
Other	14,849	6,653
Total mortgage banking income	10,462	4,502
Other noninterest income (loss)	8,404	5,340
Total noninterest income	18,866	9,842
Noninterest expense	35,388	32,592
Income (loss) before income taxes	6,429	2,423
Provision for income taxes	1,705	356
Net income	4,724	2,067
Total assets at year end	4,101,206	3,655,996	$ 4,058,921
Balance, end of year	69,802	69,635
Operating Segments | HarborOne Bank
Segment Reporting Information
Net interest and dividend income (expense)	26,510	26,419
Provision for loan losses	3,749	857
Net interest and dividend income, after provision for loan losses	22,761	25,562
Mortgage banking income:
Changes in mortgage servicing rights fair value	(1,170)	(570)
Other	(49)	222
Total mortgage banking income	(1,219)	(348)
Other noninterest income (loss)	8,526	5,352
Total noninterest income	7,307	5,004
Noninterest expense	24,288	24,865
Income (loss) before income taxes	5,780	5,701
Provision for income taxes	1,601	1,446
Net income	4,179	4,255
Total assets at year end	3,963,011	3,659,586
Balance, end of year	59,042	58,875
Operating Segments | HarborOne Mortgage
Segment Reporting Information
Net interest and dividend income (expense)	281	109
Net interest and dividend income, after provision for loan losses	281	109
Mortgage banking income:
Changes in mortgage servicing rights fair value	(3,217)	(1,581)
Other	14,898	6,431
Total mortgage banking income	11,681	4,850
Other noninterest income (loss)	(122)	(12)
Total noninterest income	11,559	4,838
Noninterest expense	10,806	7,352
Income (loss) before income taxes	1,034	(2,405)
Provision for income taxes	239	(845)
Net income	795	(1,560)
Total assets at year end	182,195	79,700
Balance, end of year	10,760	10,760
Operating Segments | HarborOne Bancorp, Inc.
Segment Reporting Information
Net interest and dividend income (expense)	(91)	(498)
Net interest and dividend income, after provision for loan losses	(91)	(498)
Mortgage banking income:
Noninterest expense	294	375
Income (loss) before income taxes	(385)	(873)
Provision for income taxes	(135)	(245)
Net income	(250)	(628)
Total assets at year end	710,316	397,183
Eliminations
Mortgage banking income:
Total assets at year end	$ (754,316)	$ (480,473)