SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment			2
Net interest and dividend income (expense)	$ 31,169	$ 27,985	$ 87,316	$ 80,728
Provision for loan losses	13,454	889	27,207	3,496
Net interest and dividend income, after provision for loan losses	17,715	27,096	60,109	77,232
Mortgage banking income:
Gain on sale of mortgage loans	34,055	11,015	77,195	24,086
Changes in mortgage servicing rights fair value	(193)	(2,474)	(5,691)	(6,866)
Other	4,281	2,964	10,962	7,442
Total mortgage banking income	38,143	11,505	82,466	24,662
Other noninterest income (loss)	6,318	5,768	19,499	18,171
Total noninterest income	44,461	17,273	101,965	42,833
Noninterest expense	45,722	36,203	124,948	103,876
Income (loss) before income taxes	16,454	8,166	37,126	16,189
Provision for income taxes	4,561	1,053	9,934	2,228
Net income	11,893	7,113	27,192	13,961
Total assets at year end	4,428,319	3,949,020	4,428,319	3,949,020	$ 4,058,921
Balance, end of year	69,802	69,635	69,802	69,635
HarborOne Bank
Mortgage banking income:
Balance, end of year	59,000		59,000
HarborOne Mortgage
Mortgage banking income:
Balance, end of year	10,800		10,800
Operating Segments | HarborOne Bank
Segment Reporting Information
Net interest and dividend income (expense)	30,599	27,855	86,248	81,296
Provision for loan losses	13,454	889	27,207	3,496
Net interest and dividend income, after provision for loan losses	17,145	26,966	59,041	77,800
Mortgage banking income:
Gain on sale of mortgage loans				1
Intersegment gain (loss)	(645)	(393)	(2,444)	(866)
Changes in mortgage servicing rights fair value	(354)	(591)	(2,014)	(1,599)
Other	334	369	1,031	1,127
Total mortgage banking income	(665)	(615)	(3,427)	(1,337)
Other noninterest income (loss)	6,326	5,772	19,640	18,191
Total noninterest income	5,661	5,157	16,213	16,854
Noninterest expense	26,300	24,405	75,806	74,527
Income (loss) before income taxes	(3,494)	7,718	(552)	20,127
Provision for income taxes	571	1,019	2,199	3,267
Net income	(4,065)	6,699	(2,751)	16,860
Total assets at year end	4,404,842	3,821,671	4,404,842	3,821,671
Balance, end of year	59,042	58,875	59,042	58,875
Operating Segments | HarborOne Mortgage
Segment Reporting Information
Net interest and dividend income (expense)	1,000	285	2,020	604
Net interest and dividend income, after provision for loan losses	1,000	285	2,020	604
Mortgage banking income:
Gain on sale of mortgage loans	34,055	11,015	77,195	24,085
Intersegment gain (loss)	645	393	2,444	866
Changes in mortgage servicing rights fair value	161	(1,883)	(3,677)	(5,267)
Other	3,947	2,595	9,931	6,315
Total mortgage banking income	38,808	12,120	85,893	25,999
Other noninterest income (loss)	(8)	(4)	(141)	(20)
Total noninterest income	38,800	12,116	85,752	25,979
Noninterest expense	19,156	11,227	48,235	27,496
Income (loss) before income taxes	20,644	1,174	39,537	(913)
Provision for income taxes	4,550	171	8,667	(256)
Net income	16,094	1,003	30,870	(657)
Total assets at year end	280,983	152,800	280,983	152,800
Balance, end of year	10,760	10,760	10,760	10,760
Operating Segments | HarborOne Bancorp, Inc.
Segment Reporting Information
Net interest and dividend income (expense)	(430)	(155)	(952)	(1,172)
Net interest and dividend income, after provision for loan losses	(430)	(155)	(952)	(1,172)
Mortgage banking income:
Noninterest expense	266	571	907	1,853
Income (loss) before income taxes	(696)	(726)	(1,859)	(3,025)
Provision for income taxes	(560)	(137)	(932)	(783)
Net income	(136)	(589)	(927)	(2,242)
Total assets at year end	729,838	693,851	729,838	693,851
Eliminations
Mortgage banking income:
Total assets at year end	$ (987,344)	$ (719,302)	$ (987,344)	$ (719,302)