SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment			2
Net interest and dividend income (expense)	$ 29,447	$ 26,713	$ 56,147	$ 52,743
Provision for loan losses	10,004	1,750	13,753	2,607
Net interest and dividend income, after provision for loan losses	19,443	24,963	42,394	50,136
Mortgage banking income:
Gain on sale of mortgage loans	30,862	8,429	43,140	13,072
Changes in mortgage servicing rights fair value	(1,111)	(2,241)	(5,498)	(4,392)
Other	4,110	2,467	6,681	4,477
Total mortgage banking income	33,861	8,655	44,323	13,157
Other noninterest income (loss)	4,777	7,063	13,181	12,403
Total noninterest income	38,638	15,718	57,504	25,560
Noninterest expense	43,838	35,081	79,226	67,673
Income (loss) before income taxes	14,243	5,600	20,672	8,023
Provision for income taxes	3,668	819	5,373	1,175
Net income	10,575	4,781	15,299	6,848
Total assets at year end	4,464,906	3,737,424	4,464,906	3,737,424	$ 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)	29,139	27,022	55,649	53,441
Provision for loan losses	10,004	1,750	13,753	2,607
Net interest and dividend income, after provision for loan losses	19,135	25,272	41,896	50,834
Mortgage banking income:
Gain on sale of mortgage loans		1		1
Intersegment gain (loss)	(1,399)	(314)	(1,799)	(473)
Changes in mortgage servicing rights fair value	(490)	(438)	(1,660)	(1,008)
Other	346	377	697	758
Total mortgage banking income	(1,543)	(374)	(2,762)	(722)
Other noninterest income (loss)	4,788	7,067	13,314	12,419
Total noninterest income	3,245	6,693	10,552	11,697
Noninterest expense	25,218	25,257	49,506	50,122
Income (loss) before income taxes	(2,838)	6,708	2,942	12,409
Provision for income taxes	27	802	1,628	2,248
Net income	(2,865)	5,906	1,314	10,161
Total assets at year end	4,451,114	3,736,515	4,451,114	3,736,515
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)	739	210	1,020	319
Net interest and dividend income, after provision for loan losses	739	210	1,020	319
Mortgage banking income:
Gain on sale of mortgage loans	30,862	8,428	43,140	13,071
Intersegment gain (loss)	1,399	314	1,799	473
Changes in mortgage servicing rights fair value	(621)	(1,803)	(3,838)	(3,384)
Other	3,764	2,090	5,984	3,719
Total mortgage banking income	35,404	9,029	47,085	13,879
Other noninterest income (loss)	(11)	(4)	(133)	(16)
Total noninterest income	35,393	9,025	46,952	13,863
Noninterest expense	18,273	8,917	29,079	16,269
Income (loss) before income taxes	17,859	318	18,893	(2,087)
Provision for income taxes	3,878	418	4,117	(427)
Net income	13,981	(100)	14,776	(1,660)
Total assets at year end	233,138	144,935	233,138	144,935
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)	(431)	(519)	(522)	(1,017)
Net interest and dividend income, after provision for loan losses	(431)	(519)	(522)	(1,017)
Mortgage banking income:
Noninterest expense	347	907	641	1,282
Income (loss) before income taxes	(778)	(1,426)	(1,163)	(2,299)
Provision for income taxes	(237)	(401)	(372)	(646)
Net income	(541)	(1,025)	(791)	(1,653)
Total assets at year end	721,153	405,776	721,153	405,776
Eliminations
Mortgage banking income:
Total assets at year end	$ (940,499)	$ (549,802)	$ (940,499)	$ (549,802)